Description:
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I think, that's good idea to add the second parameter to ctype_alnum() function.
See: bool ctype_alnum ( string text [, string register] )
Default "i". Also may be "u" and "l".
i - ignore register
u - uppercase
l - lowercase

ctype_alnum($string, "i") equivalent preg_match("/^[0-9A-z]*$/", $string)
ctype_alnum($string, "u") equivalent preg_match("/^[0-9A-Z]*$/", $string)
ctype_alnum($string, "l") equivalent preg_match("/^[0-9a-z]*$/", $string)

Sorry for my bad english.