Description:
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Documentation for key() function suggests it works only for associative arrays, however it works for both numerical and associative.

PHP Manual for key() function states for the short description:

key ? Fetch a key from an associative array

Reproduce code:
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<?php

$arr = array("test", "test2");
reset($arr);
echo key($arr);
next($arr);
echo key($arr);

?>

Expected result:
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key ? Fetch a key from an array

Actual result:
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key ? Fetch a key from an associative array