That's not an error - it's normal behavior. It just informs you that once started the in/output is removed from your console.
To avoid the message you need to start it like
nohup php server1.php </dev/null &>/dev/null &
If you want to see the output of the executed command,You need to follow this step,execute the command $tailf nohup.out
Please note : Nohup command creates a file with name nohup.out in same location where you have executed the nohup command .