<?php
$str="Please wait for sometime too, the area code is for 18 and phone number is too 5897 for";
$str=preg_replace('#(\d)\s*for#','${1}4',$str);
$str=preg_replace('#(\d)\s*too#','${1}2',$str);
$str=preg_replace('#for\s*(\d)#','4${1}',$str);
$str=preg_replace('#too\s*(\d)#','2${1}',$str);
echo $str;