According to the sources, then the only time array_reverse() would return NULL would also emit an E_WARNING in the 5.2 branch:

2696 	if (Z_TYPE_PP(input) != IS_ARRAY) {
 
2697 		php_error_docref(NULL TSRMLS_CC, E_WARNING, "The argument should be an array");
 
2698 		return;
 
2699 	}

Sorry for not mentioning that it does a warning as well:

So it does return NULL and it does a warning.

According to the documentation it does return array only, not NULL.

If you pass an unexpected type to some functions in 5.2.x, they will emit a warning and return "undefined". 5.3.x is more strict and won't silently convert incompatible types, and also returns "undefined". So this report is still bogus.

Thank you for clarification, I did not read the NULL as undefined. But what you write makes pretty much sense, I will adopt for other function return values as well.