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<?php $array = [ [ "referrer_id" => "usr157" ], [ "referrer_id" => "usr42" ] ]; $flatten = array_map(function($item) { return array_values($item)[0]; }, $array); var_dump($flatten);
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.34, 8.2.0 - 8.2.32, 8.3.0 - 8.3.32, 8.4.1 - 8.4.23, 8.5.0 - 8.5.8
array(2) { [0]=> string(6) "usr157" [1]=> string(5) "usr42" }

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