Description:
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From manual page: http://www.php.net/function.ctype-digit
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The above manual page contains a note that states: "This function requires a 
string to be useful, so for example passing in an integer will always return 
FALSE."

This is not true, as e.g. calling ctype_digit(48) will return TRUE. 

As explained elsewhere in the manual for all ctype_* functions:
"if you pass an integer smaller than 256 it will use the ASCII value 
of it to see if it fits in the specified range (digits are in 0x30-0x39). If the 
number is between -128 and -1 inclusive then 256 will be added and the check 
will be done on that."