Description:
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floor() return double type, but in documentation is floor.
This is first bug. But for it exist another bug report.

Second problem is array_key_exists() it hasn't accepted by array_key_exists, but $a[$key] = 'value' is OK. I think, that PHP accept double a key index, array_key_exists should be accept it too.

Reproduce code:
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$key = floor(1/10);
$a = array();
$a[$key] = 'value';
echo $a[$key] . "\n";
echo array_key_exists($key, $a)? "YES\n" : "NO\n";
echo gettype($key) . "\n";  

Expected result:
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value
YES
float (according documentation)

Actual result:
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value
Warning:  array_key_exists() function.array-key-exists: The first argument should be either a string or an integer in ...
NO
double