function MyStrRev()
 {
     return strRevFunction($this->string);
 }

<?php 
$string = "sanjida";
function strRevFunction($string)
{
    $string1 = "";
    $length = strlen($string);
    // echo $length."<br>";
    $i = $length;
    while ($i > 0) {
        $i--;
        $string1 .= $string[$i];
    }
    // echo $string1;
    return $string1;
}
$result = strRevFunction($string);
echo $result;