Description:
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1. Different default value for $search_value in doc
http://ru2.php.net/manual/ru/function.array-keys.php
http://ru2.php.net/manual/en/function.array-keys.php

$search_value not work 'null'

Test script:
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$result1 = array_keys(['id' => 1]); // ['id']
$result2 = array_keys(['id' => 1], null); // []

$expected = ['id'];
echo $expected === $result1 ? 1 : 0; // 1 true
echo $expected === $result2 ? 1 : 0; // 0 false


Expected result:
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['id']

Actual result:
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[]