Unaccent Special characters and bulk-rename
Renaming a file in AppleScript can be performed through the Finder:

tell application "Finder"
   set the name of file "Monterey" to "Eden"
end tell


Reducing a filename to a-z is tricky. If you are comfortable with perl, there is an ideal module called Text::Unidecode. Other approaches, such as using regular expressions, is discussed by Perl Monks in removing accents.

You mention using php, so this question may provide easier answers to integrate into your workflow, How do I remove accents from characters in a PHP string? The highest voted answer suggests:

function stripAccents($stripAccents){
  return strtr($stripAccents,'àáâãäçèéêëìíîïñòóôõöùúûüýÿÀÁÂÃÄÇÈÉÊËÌÍÎÏÑÒÓÔÕÖÙÚÛÜÝ','aaaaaceeeeiiiinooooouuuuyyAAAAACEEEEIIIINOOOOOUUUUY');
}