-------------------a.php--------------------
<?php
    $OUTPUT = shell_exec("/usr/bin/python3 python.py");
    if(strcasecmp($OUTPUT,"INVALID")){
        echo "SOMETHING";
    }else{
        echo "DOESN'T WORK";
    }
?>
--------------------------------------------

----------------------python.py-------------
print("NOT INVALID") #if you don't know python, this basically just print out "NOT INVALID" into the stdout