In general PHP instanceof and is_a () function is the same, the difference is due to the different versions of PHP, some PHP version of the is_a () to give a detailed analysis of the following.
Both the instanceof operator and the Is_a () method determine whether an object belongs to the class or the class is the parent of the object (used to determine whether a PHP variable belongs to an instance of a class of classes)
If yes, return TRUE, not return FALSE
The instanceof operator is introduced in PHP 5. Prior to this use Is_a (), but later is_a () was discarded and replaced with instanceof.
PHP 5.3.0, and restore the use of is_a ().
Now the service environment of PHP is commonly used php5.0+, so try to use instanceof instead of is_a ()
In summary, if you do not know your server environment, then recommend that you use instanceof, so as not to cause unnecessary trouble