<?php
class parse_AR()
{
	function parse(&$data)
	{
		//do stuff to $data based on AR rules
	}
}

class parse_AP()
{
	function parse(&$data)
	{
		//do stuff to $data based on AP rules
	}
}

switch ($filename)
{
	case "ar";
		$parser = new parse_AR();
		break;
	case "ap";
		$parser = new parse_AP();
		break;
	default:
		die("unsupported file");
}

$f = fopen(...);
while(...)
{
	$data = ....
	....
	$parser->parse($data);
}

fclose...

?>