<?php
$var1 = 0;
$var2 = 1;

function noreference($var1, $var2) {
    print "noreference\n";
    print $var1 + $var2."\n";
}

function reference(&$var1, &$var2) {
    print "reference\n";
    print $var1 + $var2."\n";
}

call_user_func('noreference', $var1, $var2);

call_user_func('reference', $var1, $var2);