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Functions declared with void as their return type must either omit their return statement altogether, or use an empty return statement. null is not a valid return value for a void function.

Void function may throw exceptions or return without a parameter.

Null and void are two different notions.

<?php

function foo(&$a) : void {
    $a++;

    // No return used here
}

$b = 0;
foo($b);

?>

Documentation

Related : Null

Added in PHP 8.0