Can the Png2wbmp return value return not only bool but also other values? The function can also judge whether it is true or false.

function luhn_checker($card_num,$sum){
 return ?  ;

You can return bool or $sum, and the function needs to judge whether it is true or not.

if(luhn_checker($card_num)){ }

PHP inside can use arrays and list reserved words to return multiple values, but it still needs to determine a single line …

function luhn_checker($card_num, $sum) {
 return array(true, $sum);
 list($ret, $sum) = luhn_checker($card_num, $num);
 if($ret)   ...

Additional: The kv array method mentioned by inside can also be solved into multiple variables by list, and the order of solved values is the same as that of definition.