Basics

    Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.

    Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: ^[a-zA-Z_\x80-\xff][a-zA-Z0-9_\x80-\xff]*$

    Note: For our purposes here, a letter is a-z, A-Z, and the bytes from 128 through 255 (0x80-0xff).

    Note: $this is a special variable that can't be assigned. Prior to PHP 7.1.0, indirect assignment (e.g. by using variable variables) was possible.

    Tip

    See also the Userland Naming Guide.

    For information on variable related functions, see the Variable Functions Reference.

    <?php
    $var     = 'Bob';
    $Var = 'Joe';
    echo     "$var, $Var"; // outputs "Bob, Joe"

    $4site = 'not yet'; // invalid; starts with a number
    $_4site = 'not yet'; // valid; starts with an underscore
    $täyte = 'mansikka'; // valid; 'ä' is (Extended) ASCII 228.
    ?>

    By default, variables are always assigned by value. That is to say, when you assign an expression to a variable, the entire value of the original expression is copied into the destination variable. This means, for instance, that after assigning one variable's value to another, changing one of those variables will have no effect on the other. For more information on this kind of assignment, see the chapter on Expressions.

    PHP also offers another way to assign values to variables: assign by reference. This means that the new variable simply references (in other words, "becomes an alias for" or "points to") the original variable. Changes to the new variable affect the original, and vice versa.

    To assign by reference, simply prepend an ampersand (&) to the beginning of the variable which is being assigned (the source variable). For instance, the following code snippet outputs 'My name is Bob' twice:

    <?php
    $foo     = 'Bob'; // Assign the value 'Bob' to $foo
    $bar = &$foo; // Reference $foo via $bar.
    $bar = "My name is $bar"; // Alter $bar...
    echo $bar;
    echo     $foo; // $foo is altered too.
    ?>

    One important thing to note is that only named variables may be assigned by reference.

    <?php
    $foo     = 25;
    $bar = &$foo; // This is a valid assignment.
    $bar = &(24 * 7); // Invalid; references an unnamed expression.

    function test()
    {
    return     25;
    }

    $bar = &test(); // Invalid.
    ?>

    It is not necessary to initialize variables in PHP however it is a very good practice. Uninitialized variables have a default value of their type depending on the context in which they are used - booleans default to false, integers and floats default to zero, strings (e.g. used in echo) are set as an empty string and arrays become to an empty array.

    Example #1 Default values of uninitialized variables

    <?php
    // Unset AND unreferenced (no use context) variable; outputs NULL
    var_dump($unset_var);

    // Boolean usage; outputs 'false' (See ternary operators for more on this syntax)
    echo $unset_bool ? "true\n" : "false\n";

    // String usage; outputs 'string(3) "abc"'
    $unset_str .= 'abc';
    var_dump($unset_str);

    // Integer usage; outputs 'int(25)'
    $unset_int += 25; // 0 + 25 => 25
    var_dump($unset_int);

    // Float usage; outputs 'float(1.25)'
    $unset_float += 1.25;
    var_dump($unset_float);

    // Array usage; outputs array(1) { [3]=> string(3) "def" }
    $unset_arr[3] = "def"; // array() + array(3 => "def") => array(3 => "def")
    var_dump($unset_arr);

    // Object usage; creates new stdClass object (see http://www.php.net/manual/en/reserved.classes.php)
    // Outputs: object(stdClass)#1 (1) { ["foo"]=> string(3) "bar" }
    $unset_obj->foo = 'bar';
    var_dump($unset_obj);
    ?>

    Relying on the default value of an uninitialized variable is problematic in the case of including one file into another which uses the same variable name. E_WARNING (prior to PHP 8.0.0, E_NOTICE) level error is issued in case of working with uninitialized variables, however not in the case of appending elements to the uninitialized array. isset() language construct can be used to detect if a variable has been already initialized.

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    User Contributed Notes 2 notes

    up
    77
    jeff dot phpnet at tanasity dot com
    13 years ago
    This page should include a note on variable lifecycle:

    Before a variable is used, it has no existence. It is unset. It is possible to check if a variable doesn't exist by using isset(). This returns true provided the variable exists and isn't set to null. With the exception of null, the value a variable holds plays no part in determining whether a variable is set.

    Setting an existing variable to null is a way of unsetting a variable. Another way is variables may be destroyed by using the unset() construct.

    <?php
    print isset($a); // $a is not set. Prints false. (Or more accurately prints ''.)
    $b = 0; // isset($b) returns true (or more accurately '1')
    $c = array(); // isset($c) returns true
    $b = null; // Now isset($b) returns false;
    unset($c); // Now isset($c) returns false;
    ?>

    is_null() is an equivalent test to checking that isset() is false.

    The first time that a variable is used in a scope, it's automatically created. After this isset is true. At the point at which it is created it also receives a type according to the context.

    <?php
    $a_bool     = true; // a boolean
    $a_str = 'foo'; // a string
    ?>

    If it is used without having been given a value then it is uninitalized and it receives the default value for the type. The default values are the _empty_ values. E.g Booleans default to FALSE, integers and floats default to zero, strings to the empty string '', arrays to the empty array.

    A variable can be tested for emptiness using empty();

    <?php
    $a     = 0; //This isset, but is empty
    ?>

    Unset variables are also empty.

    <?php
    empty($vessel); // returns true. Also $vessel is unset.
    ?>

    Everything above applies to array elements too.

    <?php
    $item     = array();
    //Now isset($item) returns true. But isset($item['unicorn']) is false.
    //empty($item) is true, and so is empty($item['unicorn']

    $item['unicorn'] = '';
    //Now isset($item['unicorn']) is true. And empty($item) is false.
    //But empty($item['unicorn']) is still true;

    $item['unicorn'] = 'Pink unicorn';
    //isset($item['unicorn']) is still true. And empty($item) is still false.
    //But now empty($item['unicorn']) is false;
    ?>

    For arrays, this is important because accessing a non-existent array item can trigger errors; you may want to test arrays and array items for existence with isset before using them.
    up
    13
    anisgazig at gmail dot com
    3 years ago
    clear concept of variable declaration rules and classification

    variable declaration rules:

    1.start with dollar sign($)
    2.first letter of variable name comes from a-zA-z_
    3.next letters of variable name comes from a-zA-Z0-9_
    4.no space,no syntex

    classification of variables:

    Variable are mainly Two types
    1.Predefined Variable
    2.User Define Variable

    Predefined Variable
    There are 12 predefined variables in php 8
    1.$GLOBALS
    2.$_SERVER
    3.$_REQUEST
    4.$_FILES
    5.$_ENV
    6.$_SESSION
    7.$_COOKIE
    8.$_GET
    9.$_POST
    10.$http_response_header
    11.$argc
    12.$argv

    User Define Variable
    User Define variable are 3 types
    1.variable scope
    2.variable variables
    3.reference variable

    Variable Scope
    variable scope are 3 types
    1.local scope
    2.global scope
    3.static variable
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