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<?php $string = 'abclkjabc'; $notRepeated = []; foreach (count_chars($string, 1) as $char => $count) { if ($count === 1) { $notRepeated[] = chr($char); } } var_export($notRepeated); echo "\nCount = " . count($notRepeated);
Output for 7.4.0 - 7.4.33, 8.0.1 - 8.0.30, 8.1.0 - 8.1.31, 8.2.0 - 8.2.26, 8.3.0 - 8.3.14, 8.4.1
array ( 0 => 'j', 1 => 'k', 2 => 'l', ) Count = 3

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73.94 ms | 406 KiB | 5 Q