Here are some open-source exchanges[1][2][3][4] (only the last one uses PHP). There is no guarantee that these projects don't have any bugs/backdoors. Unless you're a Senior developer and have a lot of experience, you probably shouldn't use them for anything other than educational purposes.

[1] https://github.com/rubykube/peatio (Active development).
[2] https://github.com/3s3s/opentrade
[3] https://github.com/blinktrade/bitex
[4] https://github.com/GMX9/Crypto-Exchange-PHP (Using Changenow.io API).