Compare key-value pairs between two flat associative arrays

I have these two associative arrays

The needle array

$a = array(
    "who" => "you", 
    "what" => "thing", 
    "where" => "place",
    "when" => "hour"
);

The haystack array

$b = array(
    "when" => "time", 
    "where" => "place", 
    "who" => "you",
    "what" => "thing"
);

I want to check if the $a has a match with the b with its exact key and value

and if each key and value from $a has an exact match in $b.... I want to increment the value of a variable $c by 1 and so on...

as we've seen from above there 3 possible match... and supposedly results to increment the value of $c by 3

$c = "3";