replace any characters that don't match

[mojodojo]

How could a take a rather large string and have php delete anything that is non alphanumeric?

[HighOrbit]

you could do this with regular expression functions like ereg_replace() or preg_replace().

try something like

$output_string = ereg_replace("[^[:alnum:]]", "", $input_string)

This should insert a null value (nothing) in place of any non-alphanumerics. I think it might also strip out spaces.