<?php

$a = array(
  'mark@test.com'=> "Mark Mian",
  'jhon@test.com'=> "John jack",
  'logon@test.com'=> "Bob Logon",
  'Stela@test.com'=> "Stela Josh",
  'json@test.com'=> "Json Josh",
  'bobby@test.com'=> "Bob Mark"
);
$r = preg_grep('~Mark~i', $a);
print_r($r);