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<?php $sentence = 'subdermatoglyphic'; $alphabet = range('a', 'z'); echo implode( array_diff( $alphabet, preg_split( '~[^a-z]*~i', $sentence, 0, PREG_SPLIT_NO_EMPTY ) ) );
Output for 7.2.0 - 7.2.31, 7.3.0 - 7.3.19, 7.4.0 - 7.4.7, 8.0.0alpha1 - alpha2
fjknqvwxz