i hope this will give you a clear idea how strcmp works internally.

<?php
$str1 = "b";
echo ord($str1); //98
echo "<br/>";
$str2 = "t";
echo ord($str2); //116
echo "<br/>";
echo ord($str1)-ord($str2);//-18
$str1 = "bear";
$str2 = "tear";
$str3 = "";
echo "<pre>";
echo strcmp($str1, $str2); // -18
echo "<br/>";
echo strcmp($str2, $str1); //18
echo "<br/>";
echo strcmp($str2, $str2); //0
echo "<br/>";
echo strcmp($str2, $str3); //4
echo "<br/>";
echo strcmp($str3, $str2); //-4
echo "<br/>";
echo strcmp($str3, $str3); // 0
echo "</pre>";
?>