I have a php script written, and I want  run it as a service/daemon in linux.
So i can use service myservice start/stop/restart
I am assuming it needs to be in the /etc/init.d/
You can take reference of any prebuilt startup script under /etc/init.d for eg httpd.
For eg you can take a reference of below:
#!/bin/bash
#
# chkconfig: 35 90 12
# description: Foo server
#
# Get function from functions library
. /etc/init.d/functions
# Start the service FOO
start() {
       initlog -c "echo -n Starting FOO server: "
    /path/to/FOO &
    ### Create the lock file ###
    touch /var/lock/subsys/FOO
    success $"FOO server startup"
    echo
}
# Restart the service FOO
stop() {
    initlog -c "echo -n Stopping FOO server: "
    killproc FOO
    ### Now, delete the lock file ###
    rm -f /var/lock/subsys/FOO
    echo
}
### main logic ###
case "$1" in
start)
    start
    ;;
stop)
    stop
    ;;
 status)
    status FOO
        ;;
 restart|reload|condrestart)
    stop
    start
    ;;
 *)
    echo $"Usage: $0 {start|stop|restart|reload|status}"
    exit 1
esac
exit 0
You can have detailed look here
