What does & sign mean in front of a variable?

Question

I'm 'dissecting' PunBB, and one of its functions checks the structure of BBCode tags and fix simple mistakes where possible:

function preparse_tags($text, &$errors, $is_signature = false)

What does the & in front of the $error variable mean?

Answer 1

It means pass the variable by reference, rather than passing the value of the variable. This means any changes to that parameter in the preparse_tags function remain when the program flow returns to the calling code.

function passByReference(&$test) {
    $test = "Changed!";
}

function passByValue($test) {
    $test = "a change here will not affect the original variable";
}

$test = 'Unchanged';
echo $test . PHP_EOL;

passByValue($test);
echo $test . PHP_EOL;

passByReference($test);
echo $test . PHP_EOL;

Output:

Unchanged

Unchanged

Changed!