Description:
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---
From manual page: http://www.php.net/function.ctype-alnum
---
There should be the following warning in the docs:

If you are passing in a number, ctype_alnum will return false for all numbers that do not represent alphanumeric character codes (see chr). 

Test script:
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<?php
$myVar = 10;
echo ctype_alnum($myVar) ? "T" : "F"; // returns F
$myVar = 48;
echo ctype_alnum($myVar) ? "T" : "F"; // returns T, because ASCII 48 = "a"
$myVar = "10";
echo ctype_alnum($myVar) ? "T" : "F"; // returns T