function getMissingAlphabetChars(string $input): array {
// Full alphabet
$alphabet = range('a', 'z');
// Normalize input: lowercase + keep only letters
$letters = preg_replace('/[^a-z]/', '', strtolower($input));
// Unique letters in the string
$present = array_unique(str_split($letters));
// Difference: alphabet minus present letters
return array_values(array_diff($alphabet, $present));
}
$missing = getMissingAlphabetChars("PHP Programming");
print_r($missing);
/*
run:
Array
(
[0] => b
[1] => c
[2] => d
[3] => e
[4] => f
[5] => j
[6] => k
[7] => l
[8] => q
[9] => s
[10] => t
[11] => u
[12] => v
[13] => w
[14] => x
[15] => y
[16] => z
)
*/
