<?php

function exemplo(array $args) {
	$pares = array('par1' => 1, 'par2' => 2);
	$args = array_merge($pares, array_intersect_key($args, $pares));
	list($par1, $par2) = array_values($args);
	
	return $par1 .' - '. $par2;
	
}

echo exemplo([]) . "\n";                         // 1 - 2
echo exemplo(['par1' => 3]) . "\n";              // 3 - 2
echo exemplo(['par1' => 3, 'par2' => 4]) . "\n"; // 3 - 4
echo exemplo(['par2' => 4]) . "\n";              // 1 - 4