Sanitize WordPress uploaded file names

sanitize_file_name.php

<?php

add_filter( 'sanitize_file_name', 'my_sanitize_file_name' );

function my_sanitize_file_name( $filename ) {
	$original_chars = array(
		'/А/','/Б/','/В/','/Г/', // cyrillic alphabet
		'/Д/','/Е/','/Ж/','/З/','/И/',
		'/Й/','/К/','/Л/','/М/','/Н/',
		'/О/','/П/','/Р/','/С/','/Т/',
		'/У/','/Ф/','/Х/','/Ц/','/Ч/',
		'/Ш/','/Щ/','/Ь/','/Ю/','/Я/',
		'/а/','/б/','/в/','/г/','/д/','/е/','/ж/',
		'/з/','/и/','/й/','/к/','/л/',
		'/м/','/н/','/о/','/п/','/р/',
		'/с/','/т/','/у/','/ф/','/х/',
		'/ц/','/ч/','/ш/','/щ/',
		'/ь/','/ю/','/я/',
		'/Ґ/','/ґ/','/Є/','/є/','/І/','/і/','/Ї/','/ї/', // ukrainian
		'/Ё/','/ё/','/Ы/','/ы/','/Ъ/','/ъ/','/Э/','/э/', // russian
		'/Ў/','/ў/', // belarusian
		'/Ä/','/ä/','/Ö/','/ö/','/Ü/','/ü/','/ß/', // german
		'/Ą/','/ą/','/Ć/','/ć/','/Ę/','/ę/','/Ł/','/ł/','/Ń/','/ń/','/Ó/','/ó/','/Ś/','/ś/','/Ź/','/ź/','/Ż/','/ż/', // polish (new unique letters)
		'/Ő/','/ő/','/Ű/','/ű/', // hungarian
		'/ě/','/š/','/č/','/ř/','/ž/','/ý/','/á/','/é/','/ď/','/ť/','/ň/','/ú/','/ů/', // czech
		'/Ě/','/Š/','/Č/','/Ř/','/Ž/','/Ý/','/Á/','/É/','/Ď/','/Ť/','/Ň/','/Ú/','/Ů/',
		'/א/','/ב/','/ג/','/ד/','/ה/','/ו/','/ז/','/ח/','/ט/','/י/','/ך/','/כ/','/ל/','/ם/','/מ/','/ן/','/נ/','/ס/','/ע/','/ף/','/פ/','/ץ/','/צ/','/ק/','/ר/','/ש/','/ת/', //hebrew
		'/À/','/Á/','/Â/','/Ã/','/Å/','/Æ/','/Ç/','/È/','/É/','/Ê/','/Ë/','/Ì/','/Í/','/Î/','/Ï/','/Ð/','/Ñ/','/Ò/','/Ô/','/Õ/','/×/','/Ø/','/Ù/','/Ú/','/Û/','/Ý/','/Þ/', // extra all (http://www.atm.ox.ac.uk/user/iwi/charmap.html)
		'/à/','/á/','/â/','/ã/','/å/','/æ/','/ç/','/è/','/é/','/ê/','/ë/','/ì/','/í/','/î/','/ï/','/ð/','/ñ/','/ò/','/ô/','/õ/','/×/','/ø/','/ù/','/ú/','/û/','/ý/','/þ/','/ÿ/','/Ÿ/',
		'/№/','/“/','/”/','/«/','/»/','/„/','/@/','/%/', // other
		'/‘/','/’/','/`/','/´/','/º/','/ª/','/[\s\+]/','/\.(?=.*\.)/', // alternative regex /\.([^.]*(\.[^.]+$)?)/
	);
	$sanitized_chars = array(
		'a','b','v','h', // cyrillic alphabet
		'd','e','zh','z','y',
		'j','k','l','m','n',
		'o','p','r','s','t',
		'u','f','h','c','ch',
		'sh','shh','','ju','ja',
		'a','b','v','h','d','e','zh',
		'z','y','j','k','l',
		'm','n','o','p','r',
		's','t','u','f','h',
		'c','ch','sh','sch',
		'','ju','ja',
		'g','g','je','je','i','i','ji','ji', // ukrainian
		'jo','jo','y','y','','','ye','ye', // russian
		'u','u', // belorussian
		'ae','ae','oe','oe','ue','ue','ss', // german
		'a','a','c','c','e','e','l','l','n','n','o','o','s','s','z','z','z','z', // polish
		'o','o','u','u', // hungarian
		'e','s','c','r','z','y','a','e','d','t','n','u','u',  // czech
		'e','s','c','r','z','y','a','e','d','t','n','u','u',
		// hebrew symbols don't have an equivalent one latin letter, replacing for first letter of each symbol to prevent too large filenames
		'a','b','g','d','h','v','z','h','t','y','f','k','l','f','m','f','n','s','a','f','p','f','t','q','r','s','t',
		//'alef','bet','gimel','dalet','he','vat','zayin','het','tet','yod','final-kaf','kaf','lamed','final-mem','mem','final-num','num','samekh','ayin','final-pe','pe','final-tsadi','tsadi','qof','resh','shin','tav', // hebrew letters
		'a','a','a','a','a','ae','c','e','e','e','e','i','i','i','i','d','n','o','o','o','x','o','u','u','u','y','p', // extra all
		'a','a','a','a','a','ae','c','e','e','e','e','i','i','i','i','d','n','o','o','o','x','o','u','u','u','y','p','y','y',
		'','','','','','','','', // other
		'','','','','o','a','-','-',
	);
	// First step of cleanup replacing characters
	$friendly_filename = preg_replace( $original_chars, $sanitized_chars, $filename );
	// At this point we should have a clean filename, but we're going to remove any character not covered by the above replacements,  just in case
	$friendly_filename = preg_replace( '/[^a-zA-Z0-9_\.-]/', '', $friendly_filename ); // allow only letters, numbers, underscores, dots, dashes
	// Finally all letters in the filename are set to lowercase to prevent issues with case insensitive systems.
	$friendly_filename = strtolower( $friendly_filename );
	return $friendly_filename;
}