Description:
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From manual page: http://www.php.net/function.array-diff-key#refsect1-function.array-diff-key-examples
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Example #1 *appears* incorrect (based on my own ignorance).

The description of 'array_diff_keys' is written in simple enough English I *should* be able to see it the way you do (sorry, but I don't).  I must have missed something.

Test script:
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Your example:
$array1 = array('blue'  => 1, 'red'  => 2, 'green'  => 3, 'purple' => 4);
$array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan'   => 8);

var_dump(array_diff_key($array1, $array2));

Expected result:
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I expected 'purple' to be included in the output because the function "Compares the keys from array1 against the keys from array2 and returns the difference."

Reading the Return Values portion of the function definition:
"Returns an array containing all the entries from array1 whose keys are not present in any of the other arrays."

'purple' is included in $array1, but not $array2 - thus a difference.

'cyan' and 'yellow', though technically a 'difference', are not in $array1 and therefore should not be returned based on the documentation.


Actual result:
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Actual results are identical to those shown in your example #1.