PHP: What does a & in front of a variable name mean?

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 $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.

function test()
{
   return 25;
}

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

Finally, While getting a reference as a parameter, It must pass a reference of a variable, not anything else. Though it's not a big problem because we know why we are using it. But It can happen many times. Here's how it works:

function change ( &$ref ) {
  $ref = 'changed';
}

$name = 'Wanda';

change ( $name ); // $name reference
change ( 'Wanda' ); // Error

echo $name; // output: changed

README (for more information)