function empty_check($var){ // Start Function
	if(is_array($var)){ // Determine if $var is an array or not
		foreach($var as $key => $value){ // If it is an array go through the array and assign to $key and $value
			$value = trim($value); // Remove any whitespace from the start and end of the value
			if(empty($value)){ // Check if its empty or not
				$empty[] = $key; // If its empty put the name of the field thats empty in $empty array.
			}
		}
	}else{ // If it isnt in an array
		$var = trim($var); // Remove any whitespace from the start and end of the value
		if(empty($var)){ // Check if its empty or not
			$empty[] = $var; // If its empty put the value into the $empty array.
		}
	}
	return $empty; // Return $empty.
}

<?php

if($_POST['submit']){
	$empty = empty_check($_POST);
	if($empty){
		echo "You have left the following fields empty<br />";
		foreach($empty as $value){
			echo $value . ", ";
		}
	}else{
		echo "All fields are filled in.";
	}
}

?>
<form method="post" action="">
	<input type="text" name="first_name" />
	<input type="text" name="last_name" />
	<input type="submit" value="Submit Button" name="submit" />
</form>