如何通过php获取句子中的字符串索引？

i have this code

<?php
$isi = $a['isi'];
$keyword = $_GET['tbSearch'];
$index = strpos("Ut varius consectetur tellus, id hendrerit neque auctor vel.","id");
echo "<h4>".$index."</h4>"          
?>

and the output is "30"

now, i have another code

<?php
$isi = $a['isi'];//from database
$keyword = $_GET['tbSearch'];//query string
$index = strpos($isi,$keyword);
echo "<h4>".$index."</h4>"          
?>

but the output is empty. any idea?

Based on the comments, you have to use stripos if you do not want to match the case, otherwise strpos will and it will return false.