$str = 'a@gmail.com';
$key = '11111';
echo strpos($str, $key);
exit;

$str = 'a@gmail.com';
$arr = array('11111' => 'test');
foreach ($arr as $key => $val)
{
    echo 'String: '.$str.'<br>';
    echo 'Key: '.$key.'<br>';
    echo 'Found at position: '.strpos($str, $key);
}
exit;

String: a@gmail.com
Key: 11111
Found at position: 2

$str = 'a@gmail.com';
$arr = array('111' => 'test');
foreach ($arr as $key => $val)
{
    echo 'String: '.$str.'<br>';
    echo 'Key: '.$key.'<br>';
    echo 'Found at position: '.strpos($str, $key);
}
exit;

String: a@gmail.com
Key: 111
Found at position: 9