Ctype

ctype is a set of functions used for character type checking. These functions are primarily designed to determine the type of characters in a string. They are useful for tasks such as input validation, data processing, and character manipulation. The ctype functions are part of PHP’s standard library.

<?php
$strings = ['KjgWZC', 'arf12'];
foreach ($strings as $testcase) {
    if (ctype_alpha($testcase)) {
        echo "The string $testcase consists of all letters.\n";
    } else {
        echo "The string $testcase does not consist of all letters.\n";
    }
}
?>

Documentation

See also https://www.sitepoint.com/an-introduction-to-ctype-functions/

Related : filter