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<?php $test = [ 'a', 'b', 'a', 'b', 'a', 'a', 'a' ]; $result = []; // The first two need to be setup manually. There should probably be a guard around this // with a length check for $test. $result[] = array_shift($test); $result[] = array_shift($test); // Inside the loop we'll keep removing items while(count($test)){ // Setup common variables $current = array_shift($test); $last = end($result); $last_char = $last[-1]; if(strlen($last) >= 2){ // If we have two (or more) characters at the end of the array, just append $result[] = $current; }elseif($current === $last_char){ // Also, if we have the same character as last time, just append $result[] = $current; }else{ // Otherwise we have a different character, so append the current to the previus $result[ count($result) - 1 ] = $last_char . $current; } } var_dump($result);