Hi! jvandyk raised the question whether += is officially supported by PHP as it is not in the documentation. I contacted Goba who pointed me to lxr.php.net and from there I read the relevant pieces of PHP source code and it's quite clear it does. Stop here if you are not interested in gory details. I said you should stop, right? OK, here are the details if you have not stopped: http://lxr.php.net/search?string=T_PLUS_EQUAL both in Zend and ZendEngine2 we can see that T_PLUS_EQUAL is executed by: zend_do_binary_assign_op(ZEND_ASSIGN_ADD, &$$, &$1, &$3 TSRMLS_CC); so we move on to http://lxr.php.net/search?string=ZEND_ASSIGN_ADD and jump to http://lxr.php.net/source/Zend/zend_execute.c#1162 case ZEND_ASSIGN_ADD: EG(binary_op) = add_function; goto binary_assign_op_addr; and finally we peek into add_function: http://lxr.php.net/source/Zend/zend_operators.c#613 617 if (op1->type == IS_ARRAY && op2->type == IS_ARRAY) { So. We gained the knowledge that it's split into an addition and an assign before the op types are checked. Therefore, if + works for a certain kind of operation, then += does, too. (Zend2 analysis is left as an exercise. It leads to add_function, too.) Regards NK