If you don't run PHP in safe mode, you can execute shell commands.
$output = shell_exec("chown webx_user.webx filename");
you can for example make filename owned by webx_user in the group webx.
Why do you want to do this though? Shouldn't the file be accessible by everyone after uploading? You wont be able to access the file with the user permissions of webx_user via PHP this way though. You should make sure the permissions are set to "chmod 666 filename" if it should be read/write accessible by everybody.
Btw, I even use PHP for shell scripting this way, because I know it better than Bash.
chmod +x scriptname
you can just execute the script like any other shell script, if you have php installed. (Just a side remark)
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