substr php showing empty in some index code example

Example 1: if part of stringp ph

$a = 'How are you?';

if (strpos($a, 'are') !== false) {
    echo 'true';
}

Example 2: substr() php

<?php
echo substr('abcdef', 1);     // bcdef
echo substr('abcdef', 1, 3);  // bcd
echo substr('abcdef', 0, 4);  // abcd
echo substr('abcdef', 0, 8);  // abcdef
echo substr('abcdef', -1, 1); // f

// Accessing single characters in a string
// can also be achieved using "square brackets"
$string = 'abcdef';
echo $string[0];                 // a
echo $string[3];                 // d
echo $string[strlen($string)-1]; // f

?>

//substr() function returns certain bits of a string