I want to start my server through nohup.php but the command is not running and displays following error
nohup:ignoring input and appending output to 'nohup.out'
nohup php server1.php
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 &