Function to validate $_POST array - Return another array - not quite there yet

function validate() {
//Returns 'false' with no errors
//Returns 'true' with errors

	$ERROR_Validate = false;
	$ERROR_Specify = array();

	foreach($_POST as $a) {
		if ($a == "") {
			$ERROR_Validate = true;
			array_push($ERROR_Specify, "$a is empty!";
		}
	}
		
	if($ERROR_Validate == false) {
		return false;
	} else {
		return array($ERROR_Specify);
	}
}

OK all i want to do here is test for empty form inputs. its working fine. Then I wanted to make it report which inputs were missing and i cant quite figure out how. Above is what i had but I realise "array_push($ERROR_Specify, "$a is empty!";" is wrong. this will add > "is empty" < to the array.

I need to figure out how to get the correct key from the $_POST array when the element is empty. Also, running this script results in a error and im not sure why. commenting out the array_push line fixes it. I dont understand why.

Thanks for reading!

Im still learning php so youll have to forgive me for some silly mistakes!