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:
For our purposes here, a letter is a-z, A-Z, and the ASCII
characters from 127 through 255 (0x7f-0xff).
$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.
In PHP 3, 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 4 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.
This also means that no copying is performed; thus, the assignment
happens more quickly. However, any speedup will likely be noticed
only in tight loops or when assigning large
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:
$foo = 'Bob'; // Assign the value 'Bob' to $foo
$bar = &$foo; // Reference $foo via $bar.
$bar = "My name is $bar"; // Alter $bar...
echo $foo; // $foo is altered too.
One important thing to note is that only named variables may be
assigned by reference.
$foo = 25;
$bar = &$foo; // This is a valid assignment.
$bar = &(24 * 7); // Invalid; references an unnamed expression.
$bar = &test(); // Invalid.