fix: update php version and optimize string operations

Updated the PHP version requirement from 8.0 to 8.1 in composer.json. Refactored the string operations functions in category.php and productList.php. Replaced the 'strpos' function with 'str_contains' for better readability and performance. Also replaced the usage of 'strpos' to check for string start with 'str_starts_with' for better readability. Fixed some strings formatting.