Debug PHP coding

PHP:
Code checker: https://phpcodechecker.com
About failures: https://kinsta.com/blog/10-things-not-to-do-in-php-7
Online SEPA Credit Transfer QR-code generator: https://epc-qr.eu/?form

Site to find many programmers tools:
https://extendsclass.com/

Variable-width encoding:
Cutting a string value using substr() can crash inside a character of several bytes. Functions named like mb_substr() can handle a character of several bytes.
Note: ‘Multibyte character set’ is not a proper name for in fact multibyte encoding.

MariaDB database:
This MySQL database does not cut off longer values anymore. Using a state-of-the-art version, your application can raise bugs.

HTML using F12:
Statements get outdated in a process of deprecation. My suggestion is to warn your application supplier in order to fix before a warning in yellow becomes an error in red.