Run a command as another user once:
sudo -u www-data php script.php
This should work if you have sudo installed and are root (or another user that is allowed to do that; see the sudo group, man sudoers and visudo).
For reusability, add an alias. Place this in your .bashrc, .profile or similar (and reload the shell to make it effective):
alias phpwww='sudo -u www-data php'
You can then type phpwww script.php and it will actually execute sudo -u www-data php script.php for you.
For other, more complex and error-prone ways, read on.
As for always running php as www-data, there are several possiblities. You could create a simple wrapper shellscript. If /usr/bin/php is only a soft-link to /usr/bin/php5 or similar, that makes it simpler. Just replace the soft-link (NOT the file php5) with a script like this:
#!/bin/sh
sudo -u www-data php5 $*
return $?
That's not tested though. Also be aware that this will ALWAYS try to run php5 as user www-data, even if the user may not be root and may not have permission to do so. And it may also not be what you really want. Some installed services may run into problems when trying to execute php.
A (possibly better) solution to only apply that to root may be to leave the soft-link /usr/bin/php alone and place the script in /root/bin instead. Then add that folder to PATH via .bashrc, .profile or similar. If you have /etc/skel/.profile, that may point out how that is done:
# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
PATH="$HOME/bin:$PATH"
fi
Once this is in your .bashrc, .profile or similar, every new shell you open will allow you to directly execute any executables (+x) in $HOME/bin (/root/bin for root).
Hint: You may want to name the wrapper script something like phpwww so you explicitly specify php script.php or phpwww script.php to decide if you want regular or sudo'ed php.
Best Answer
Use
xargs -por GNUparallelto speed things up considerably.