Optimization of php code 7.4.4

Hello, our project is in php and 7.4.4, we need to optimize the old code (it was written about 7-8 years ago) for much less load on the server and the database. Just need to optimize the PHP files 18 PCs (accesses to the server will provide).

PS to superfluous questions did not arise: the database Itself is already optimized SQL programmer with 12 years of experience, the server itself optimized sysadmin with 10 years of experience.