Well, that works if "77005,77006,77007,77008,77009" is the string itself. But that's not likely to be what you want. More likely:
$strZipcodes = '77005,77006,77007,77008,77009';
$strZipcodes = "'" . str_replace(',', "','", $strZipcodes) . "'";
// ...
WHERE $zip IN ($strZipcodes);