<?php
$str = 'z1.z2.z3.z6;
$z = array();

$z[0] = '/z1./';
$z[1] = '/z2./';
$z[2] = '/z3./';
$z[3] = '/z6/';

$replaced = array();

$replacements[3] = '/[f1]/';
$replacements[2] = '/[f2]/';
$replacements[1] = '/[f3]/';
$replacements[0] = '/[f6]/';

echo preg_replace($z, $replacements, $str);
?>