You can try searching online for malicious PHP scripts which have been found on compromised servers and then shared with the community but I doubt you will find much.
There's definitely going to be a collection of YARA rules out there for numerous malicious PHP scripts, so you could try doing a YARA search on VirusTotal with the signatures you find - you'll need access to VirusTotal Intelligence to do this as well as download anything you find.
You can also try CrowdStrike's MalQuery service but it is also a paid offering and is targeted to enterprise, not home users, like VirusTotal Intelligence.