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<?php $arr1 = array('1', '2', '3', '4', '5', '6', '7'); $arr2 = array('a', 'b', 'c', 'd', 'e'); $result = []; foreach($arr1 as $key => $value){ $result[] = $value; if((0 == ($key + 1) % 3) && isset($arr2[($key + 1)/3-1])){ $result[] = $arr2[($key + 1)/3-1]; } } print_r($result);

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