Reference explanation from officicial PHP site (https://www.php.net/manual/en/) and some from w3schools (https://w3schools.com/php). Not complete and still undergoing review. It contains more detailed explanation. Go to https://github.com/Fortunate17Adunbi/php to view code and learn
Fortunate17Adunbi/PHP_REFERENCE
Reference explanation from officicial PHP site https://www.php.net/manual/en/