PHP Point of Sale (Open Source version) or InvoicePlane.
Imagine you downloaded "PHP POS System v1.5" from a questionable source. You get a "500 Internal Server Error." Here is the professional fix workflow: php point of sale source code fix download
This updates only the affected lines of php point of sale source code without a full re-download. PHP Point of Sale (Open Source version) or InvoicePlane
For those looking to study or repair existing systems, platforms like SourceForge CodeCanyon are the primary hubs. Open Source: the code will break.
Many free POS scripts were written for PHP 5.6. If you are running a modern server with PHP 8.0+, the code will break.