COMP284 Scripting Languages – Handouts
COMP284 Scripting Languages
Lecture 11: PHP (Part 3)
Handouts
Ullrich Hustadt
Department of Computer Science
School of Electrical Engineering, Electronics, and Computer Science
University of Liverpool
Contents
1 Special types
NULL
Resources
2 Control structures
Conditional statements
Switch statements
While- and Do While-loops
For-loops
3 Functions
Defining a function
Calling a function
Variables
Functions and HTML
Variable-length argument lists
4 PHP libraries
Include/Require
COMP284 Scripting Languages Lecture 11 Slide L11 – 1
Special types NULL
NULL
• NULL is both a special type and a value
• NULL is the only value of type NULL
and the name of this constant is case-insensitive
• A variable has both type NULL and value NULL in the following three
situations:
1 The variable has not yet been assigned a value (not equal to NULL)
2 The variable has been assigned the value NULL
3 The variable has been unset using the unset operation
• There are a variety of functions that can be used to test whether a
variable is NULL including:
• bool isset($variable)
TRUE iff $variable exists and does not have value NULL
• bool is_null(expr)
TRUE iff expr is identical to NULL
COMP284 Scripting Languages Lecture 11 Slide L11 – 2
Special types NULL
NULL
Warning: Using NULL with == may lead to counter-intuitive results
$d = array();
echo var_dump($d), “\n”;
array(0) {
}
echo ’is_null($d): ’, (is_null($d)) ? “TRUE\n”: “FALSE\n”;
is_null($d): FALSE
echo ’$d === null: ’, ($d === null) ? “TRUE\n”: “FALSE\n”;
$d === null: FALSE
echo ’$d == null: ’, ($d == null) ? “TRUE\n”: “FALSE\n”;
$d == null: TRUE
Type juggling means that an empty array is (loosely) equal to NULL
but not identical (strictly equal) to NULL
COMP284 Scripting Languages Lecture 11 Slide L11 – 3
Special types Resources
Resources
A resource is a reference to an external resource and corresponds to
a Perl filehandle
• resource fopen(filename, mode)
Returns a file pointer resource for filename access using mode on
success, or FALSE on error
Mode Operation Create Truncate
’r’ read file
’r+’ read/write file
’w’ write file yes yes
’w+’ read/write file yes yes
’a’ append file yes
’a+’ read/append file yes
’x’ write file yes
’x+’ read/write file yes
See http://www.php.net/manual/en/resource.php for further details
COMP284 Scripting Languages Lecture 11 Slide L11 – 4
http://www.php.net/manual/en/resource.php
Special types Resources
Resources
• bool fclose(resource)
• Closes the resource
• Returns TRUE on success
• string fgets(resource [, length])
• Returns a line read from resource and
returns FALSE if there is no more data to be read
• With optional argument length, reading ends when length− 1 bytes
have been read, or a newline or on EOF (whichever comes first)
• string fread(resource,length)
• Returns length characters read from resource
$handle = fopen(’somefile.txt’, ’r’);
while ($line = fgets($handle )) {
// processing the line of the file
}
fclose($handle );
COMP284 Scripting Languages Lecture 11 Slide L11 – 5
Special types Resources
Resources
• int fwrite(resource, string [, length])
• Writes a string to a resource
• If length is given, writing stops after length bytes have been written or the
end of string is reached, whichever comes first
• int fprintf(resource, format, arg1, arg2, …)
• Writes a list of arguments to a resource in the given format
• Identical to fprintf with output to resource
• int vfprintf (resource, format, array)
• Writes the elements of an array to a resource in the given format
• Identical to vprintf with output to resource
$handle = fopen(’somefile.txt’, ’w’);
fwrite($handle ,”Hello World!”.PHP_EOL ); // ‘logical newline ’
fclose($handle );
In contrast to Perl, in PHP \n always represents the character with ASCII
code 10 not the platform dependent newline ; use PHP_EOL instead
COMP284 Scripting Languages Lecture 11 Slide L11 – 6
Control structures Conditional statements
Control structures: conditional statements
The general format of conditional statements is very similar but not
identical to that in Java and Perl:
if (condition) {
statements
} elseif (condition) {
statements
} else {
statements
}
• the elseif-clauses is optional and there can be more than one
Note: elseif instead of elsif!
• the else-clause is optional but there can be at most one
• in contrast to Perl, the curly brackets can be omitted if there is only
a single statement in a clause
COMP284 Scripting Languages Lecture 11 Slide L11 – 7
Control structures Conditional statements
Control structures: conditional statements/expressions
• PHP allows to replace curly brackets with a colon : combined with an
endif at the end of the statement:
if (condition ):
statements
elseif (condition ):
statements
else:
statements
endif
This also works for the switch statement in PHP
However, this syntax becomes difficult to parse
when nested conditional statements are used and is best avoided
• PHP also supports conditional expressions
condition ? if_true_expr : if_false_expr
COMP284 Scripting Languages Lecture 11 Slide L11 – 8
Control structures Switch statements
Control structures: switch statement
A switch statement in PHP takes the following form
switch (expr) {
case expr1:
statements
break;
case expr2:
statements
break;
default:
statements
break;
}
• there can be arbitrarily many case-clauses
• the default-clause is optional but there can be
at most one
• expr is evaluated only once and then compared
to expr1, expr2, etc using (loose) equality ==
• once two expressions are found to be equal the
corresponing clause is executed
• if none of expr1, expr2, etc are equal to expr,
then the default-clause will be executed
• break ‘breaks out’ of the switch statement
• if a clause does not contain a break command,
then execution moves to the next clause
COMP284 Scripting Languages Lecture 11 Slide L11 – 9
Control structures Switch statements
Control structures: switch statement
Example:
switch ($command) {
case “North”:
$y += 1; break;
case “South”:
$y -= 1; break;
case “West”;
$x -= 1; break;
case “East”:
$x += 1; break;
case “Search”:
if (($x = 5) && ($y = 3))
echo “Found a treasure\n”;
else
echo “Nothing here\n”;
break;
default:
echo “Not a valid command\n”; break;
}
COMP284 Scripting Languages Lecture 11 Slide L11 – 10
Control structures Switch statements
Control structures: switch statement
Not every case-clause needs to have associated statements
Example:
switch ($month) {
case 1: case 3: case 5: case 7:
case 8: case 10: case 12:
$days = 31;
break;
case 4: case 6: case 9: case 11:
$days = 30;
break;
case 2:
$days = 28;
break;
default:
$days = 0;
break;
}
COMP284 Scripting Languages Lecture 11 Slide L11 – 11
Control structures While- and Do While-loops
Control structures: while- and do while-loops
• PHP offers while-loops and do while-loops
while (condition) {
statements
}
do {
statements
} while (condition );
• As usual, curly brackets can be omitted if the loop consists of only one
statement
Example:
// Compute the factorial of $number
$factorial = 1;
do {
$factorial *= $number –;
} while ($number > 0);
COMP284 Scripting Languages Lecture 11 Slide L11 – 12
Control structures For-loops
Control structures: for-loops
• for-loops in PHP take the form
for (initialisation; test; increment) {
statements
}
Again, the curly brackets are not required if the body of the loop only
consists of a single statement
• In PHP initialisation and increment can consist of more than one
statement, separated by commas instead of semicolons
Example:
for ($i = 3, $j = 3; $j >= 0; $i++, $j –)
echo “$i – $j – “, $i*$j, “\n”;
3 – 3 – 9
4 – 2 – 8
5 – 1 – 5
6 – 0 – 0
COMP284 Scripting Languages Lecture 11 Slide L11 – 13
Control structures For-loops
Control structures: break and continue
• The break command can also be used in while-, do while-, and for-loops
and discontinues the execution of the loop
while ($value = array_shift($data) {
$written = fwrite($resource ,$value );
if (! $written) break;
}
• The continue command stops the execution of the current iteration of a
loop and moves the execution to the next iteration
for ($x = -2; $x <= 2; $x++) { if ($x == 0) continue; printf("10 / %2d = %3d\n",$x ,(10/ $x)); } 10 / -2 = -5 10 / -1 = -10 10 / 1 = 10 10 / 2 = 5 COMP284 Scripting Languages Lecture 11 Slide L11 – 14 Functions Defining a function Functions Functions are defined as follows in PHP: function identifier($param1 ,&$param2 , ...) { statements } • Functions can be placed anywhere in a PHP script but preferably they should all be placed at start of the script (or at the end of the script) • Function names are case-insensitive • The function name must be followed by parentheses • A function has zero, one, or more parameters that are variables • Parameters can be given a default value using $param = const_expr • When using default values, any defaults must be on the right side of any parameters without defaults COMP284 Scripting Languages Lecture 11 Slide L11 – 15 Functions Defining a function Functions Functions are defined as follows in PHP: function identifier($param1 ,&$param2 , ...) { statements } • The return statement return value can be used to terminate the execution of a function and to make value the return value of the function • The return value does not have to be scalar value • A function can contain more than one return statement • Different return statements can return values of different types COMP284 Scripting Languages Lecture 11 Slide L11 – 16 Functions Calling a function Calling a function A function is called by using the function name followed by a list of arguments in parentheses function identifier($param1 , &$param2 , ...) { ... } ... identifier(arg1 , arg2 ,...) ... • The list of arguments can be shorter as well as longer as the list of parameters • If it is shorter, then default values must have been specified for the parameters without corresponding arguments Example: function sum($num1 ,$num2) { return $num1+$num2; } echo "sum: ",sum(5,4),"\n"; $sum = sum (3 ,2); COMP284 Scripting Languages Lecture 11 Slide L11 – 17 Functions Variables Variables PHP distinguishes three categories of variables: • Local variables are only accessible in the part of the code in which they are introduced • Global variables are accessible everywhere in the code • Static variables are local variables within a function that retain their value between separate calls of the function By default, variables in PHP are local but not static (Variables in Perl are by default global) COMP284 Scripting Languages Lecture 11 Slide L11 – 18 Functions Variables PHP functions: Example function bubble_sort($array) { // $array , $size , $i, $j are all local if (! is_array($array )) trigger_error("Argument not an array\n", E_USER_ERROR ); $size = count($array ); for ($i=0; $i <$size; $i++) { for ($j=0; $j <$size -1-$i; $j++) { if ($array[$j+1] < $array[$j]) { swap($array , $j, $j+1); } } } return $array; } function swap(&$array , $i, $j) { // swap expects a reference (to an array) $tmp = $array[$i]; $array[$i] = $array[$j]; $array[$j] = $tmp; } COMP284 Scripting Languages Lecture 11 Slide L11 – 19 Functions Variables PHP functions: Example function bubble_sort($array) { ... swap($array , $j , $j+1); ... return $array; } function swap(&$array , $i, $j) { $tmp = $array[$i]; $array[$i] = $array[$j]; $array[$j] = $tmp; } $array = array (2,4,3,9,6,8,5,1); echo "Before sorting ", join(", ",$array), "\n"; $sorted = bubble_sort($array ); echo "After sorting ", join(", ",$array), "\n"; echo "Sorted array ", join(", ",$sorted), "\n"; Before sorting 2, 4, 3, 9, 6, 8, 5, 1 After sorting 2, 4, 3, 9, 6, 8, 5, 1 Sorted array 1, 2, 3, 4, 5, 6, 8, 9 COMP284 Scripting Languages Lecture 11 Slide L11 – 20 Functions Variables Functions and global variables • A variable is declared to be global using the keyword global function echo_x($x) { echo $x," "; global $x; echo $x; } $x = 5; // this is a global variable called $x echo_x (10); // prints first ‘10’ then ‘5’ ; an otherwise local variable is made accessible outside its normal scope using global ; all global variables with the same name refer to the same storage location/data structure ; an unset operation removes a specific variable, but leaves other (global) variables with the same name unchanged COMP284 Scripting Languages Lecture 11 Slide L11 – 21 Functions Variables PHP functions and Global variables function modify_or_destroy_var($arg) { global $x , $y; if (is_bool($arg) && !$arg) { $x = $x * $y; } if (is_bool($arg) && $arg) { unset($x); echo $x; } } $x = 2; $y = 3; $z = 4; echo "1: \$x = $x, \$y = $y, \$z = $z\n"; 1: $x = 2, $y = 3, $z = 4 unset($z); echo "2: \$x = $x, \$y = $y, \$z = $z\n"; PHP Notice: Undefined variable: z in script on line 9 2: $x = 2, $y = 3, $z = modify_or_destroy_var(false ); echo "3: \$x = $x, \$y = $y\n"; 3: $x = 6, $y = 3 modify_or_destroy_var(true); echo "4: \$x = $x, \$y = $y\n"; PHP Notice: Undefined variable: x in script on line 4 4: $x = 6, $y = 3 COMP284 Scripting Languages Lecture 11 Slide L11 – 22 Functions Variables PHP functions and Static variables • A variable is declared to be static using the keyword static and should be combined with the assignment of an initial value (initialisation) function counter () { static $count = 0; return $count ++; } ; static variables are initialised only once 1 function counter () { static $count = 0; return $count ++; } 2 $count = 5; 3 echo "1: global \$count = $count\n"; 4 echo "2: static \$count = ",counter(),"\n"; 5 echo "3: static \$count = ",counter(),"\n"; 6 echo "4: global \$count = $count\n"; 1: global $count = 5 2: static $count = 0 3: static $count = 1 4: global $count = 5 COMP284 Scripting Languages Lecture 11 Slide L11 – 23 Functions Functions and HTML Functions and HTML • It is possible to include HTML markup in the body of a function definition • The HTML markup can in turn contain PHP scripts • A call of the function will execute the PHP scripts, insert the output into the HTML markup, then output the resulting HTML markup
COMP284 Scripting Languages Lecture 11 Slide L11 – 24
Functions Variable-length argument lists
Functions with variable number of arguments
The number of arguments in a function call is allowed to exceed the
number of its parameters
; the parameter list only specifies the minimum number of arguments
• int func_num_args()
returns the number of arguments passed to a function
• mixed func_get_arg(arg_num)
returns the specified argument, or FALSE on error
• array func_get_args()
returns an array with copies of the arguments passed to a function
function sum() { // no minimum number of arguments
if (func_num_args () < 1) return null; $sum = 0; foreach (func_get_args () as $value) { $sum += $value; } return $sum; } COMP284 Scripting Languages Lecture 11 Slide L11 – 25 PHP libraries Include/Require Including and requiring files • It is often convenient to build up libraries of function definitions, stored in one or more files, that are then reused in PHP scripts • PHP provides the commands include, include_once, require, and require_once to incorporate the content of a file into a PHP script include ’mylibrary.php’; • PHP code in a library file must be enclosed within a PHP start tag
• The incorporated content inherits the scope of the line in which an
include command occurs
• If no absolute or relative path is specified, PHP will search for the file
• first, in the directories in the include path include_path
• second, in the script’s directory
• third, in the current working directory
COMP284 Scripting Languages Lecture 11 Slide L11 – 26
PHP libraries Include/Require
Including and requiring files
• Several include or require commands for the same library file
results in the file being incorporated several times
; defining a function more than once results in an error
• Several include_once or require_once commands for the same
library file results in the file being incorporated only once
• If a library file requested by include and include_once cannot be
found, PHP generates a warning but continues the execution of the
requesting script
• If a library file requested by require and require_once cannot be
found, PHP generates a error and stops execution of the requesting
script
COMP284 Scripting Languages Lecture 11 Slide L11 – 27
PHP libraries Include/Require
PHP Libraries: Example
mylibrary.php
example.php
COMP284 Scripting Languages Lecture 11 Slide L11 – 28
PHP libraries Include/Require
Revision
Read
• Chapter 4: Expressions and Control Flow in PHP
• Chapter 5: PHP Functions and Objects
• Chapter 7: Practical PHP
of
R. Nixon:
Learning PHP, MySQL, and JavaScript.
O’Reilly, 2009.
• http://uk.php.net/manual/en/language.control-structures.php
• http://uk.php.net/manual/en/language.functions.php
• http://uk.php.net/manual/en/function.include.php
• http://uk.php.net/manual/en/function.include-once.php
• http://uk.php.net/manual/en/function.require.php
• http://uk.php.net/manual/en/function.require-once.php
COMP284 Scripting Languages Lecture 11 Slide L11 – 29
http://uk.php.net/manual/en/language.control-structures.php
http://uk.php.net/manual/en/language.functions.php
http://uk.php.net/manual/en/function.include.php
http://uk.php.net/manual/en/function.include-once.php
http://uk.php.net/manual/en/function.require.php
http://uk.php.net/manual/en/function.require-once.php
Lecture 11
Special types
NULL
Resources
Control structures
Conditional statements
Switch statements
While- and Do While-loops
For-loops
Functions
Defining a function
Calling a function
Variables
Functions and HTML
Variable-length argument lists
PHP libraries
Include/Require