How to use strlen?

Answer 1

strlen() returns the number of bytes rather than the number of characters in a string.

Example

<?php
$str = 'abcdef';
echo strlen($str); // 6
$str = ' ab cd ';
echo strlen($str); // 7
?>

Answer 2

how does it differ in strpos?

Answer 3

Find the position of the first occurrence of a substring in a string

Example
<?php
$mystring = 'abc';
$findme = 'a';
$pos = strpos($mystring, $findme);

// Note our use of ===. Simply == would not work as expected
// because the position of 'a' was the 0th (first) character.

if ($pos === false) {
echo "The string '$findme' was not found in the string '$mystring'";
} else {
echo "The string '$findme' was found in the string '$mystring'";
echo " and exists at position $pos";
}
?>

Sorry for late reply.