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_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'
Note: For our purposes here, a letter is a-z, A-Z, and the ASCII characters from 127 through 255
$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.
As of PHP 4, PHP 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 arrays or objects.
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...