Description:
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Tested on PHP 5.2.9
When trying to check if a key exists into an array, and the key is the standard representation of an integer, php must interpreted as such like the documentation says: (i.e. "8" will be interpreted as 8, while "08" will be interpreted as "08")
This don't happens when the value of the key is the same than the maximum value of an integer (PHP_INT_MAX).
Assuming that PHP_INT_MAX is 2147483647, the two lines below will not return true as expected.
isset($array[2147483647]); //true
isset($array['2147483647']); //false

This doesn't happens when key is smaller than the maximum value of an integer.
isset($array[2147483646]); //true
isset($array['2147483646']); //true

Test script:
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<?php
$maxInteger = (int)PHP_INT_MAX;
$keyInt = $maxInteger;
$keyStr = strval($keyInt);

echo "Maximum value is $maxInteger \n";

$arr = array($keyInt => 1);


echo 'Key Integer exists: ' . (int)isset($arr[$keyInt]) . "\n"; //Yes
echo 'Key String exists: ' . (int)isset($arr[$keyStr]) . "\n"; //No

$keyInt = $maxInteger - 1;
$keyStr = strval($keyInt);

echo 'Key Integer exists: ' . (int)isset($arr[$keyInt]) . "\n"; //Yes
echo 'Key String exists: ' . (int)isset($arr[$keyStr]) . "\n"; //Yes