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<?php class C { function foo(&$a, $b) { $a = 'modified'; $b = 'modified'; } } class S { static function foo(&$a, $b) { $a = 'modified'; $b = 'modified'; } } $a = $b = 'orig'; $f = array(new C, 'foo'); $f($a, $b); assert($a === 'modified'); assert($b === 'orig'); $a = $b = 'orig'; $f = array('S', 'foo'); $f($a, $b); assert($a === 'modified'); assert($b === 'orig'); $a = $b = 'orig'; $f = 'S::foo'; $f($a, $b); assert($a === 'modified'); assert($b === 'orig'); $a = $b = 'orig'; $f = function(&$a, $b) { $a = 'modified'; $b = 'modified'; }; $f($a, $b); assert($a === 'modified'); assert($b === 'orig');