Is there a specific PHP library or function that may affect the processing of the string "esse" in this context?

The issue may be related to the use of the `mb_strpos` function in PHP to find the position of a substring within a string. This function is sensitive to multibyte characters, which may cause unexpected results when processing certain strings. To solve this issue, you can use the `mb_stripos` function instead, which performs a case-insensitive search for a substring within a string while taking multibyte characters into account.

$string = "Esse";
$substring = "esse";
$position = mb_stripos($string, $substring);

if ($position !== false) {
    echo "Substring found at position: " . $position;
} else {
    echo "Substring not found";
}