Linux – Using rc.local to run a command on boot

Try putting /bin/bash in front of your command. You can't use relative paths because there is no environment to keep track of them. If you run everything in a bash-shell, bash will keep track of these things for you. Same goes for crontab

This task is well suited to a process monitor like Monit.

The monitoring configuration will be made much easier with Monit if your PHP process produces a PID file. An article describing how to use Monit to check and start PHP processes is available here.