Description:
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Function array_slice() always preserve string keys. The parameter "preserve_keys" is necessary only for numerical keys. The documentation is silent about it.

Reproduce code:
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$form = array('one' => 'one value', 'two' => 'two value', 'three' => 'three value');
print_r(array_slice($form, 2));

Expected result:
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Array ( [0] => three value )

Actual result:
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Array ( [three] => three value )