I want to add some cases to what Sbastian wrote some years ago, and cumulated with other user that says the comparison is case insensitive (php 4).

Tested on PHP 7.4 -> 8.4

You can see that PHP 8 will vanish integer 1 from result if there are a boolean true value when the flag is SORT_REGULAR.

<?php  declare(strict_types=1); // it works the same without it

$a = [true, false, 'true', 'false', 1, null, '', '0', '123', 0, 123, 'AaA', 'AAA'];

foreach (['SORT_REGULAR', 'SORT_NUMERIC', 'SORT_STRING', 'SORT_LOCALE_STRING'] as $flag) {
    $unique = array_unique($a, constant($flag));

    $acc = '';
    foreach ($unique as $k => $u) {
        $type = gettype($u);
        $formattedVal = is_bool($u) ? ($u ? 'true' : 'false') : (is_string($u) ? "'$u'" : $u);
        $acc .= "$type(" . $formattedVal  . ') | ';
    }
    
     echo "{$flag} ==> " . trim($acc, ' | ') . PHP_EOL;
    
}

// PHP 7.4
// SORT_REGULAR ==> boolean(true) | boolean(false) | integer(1) | string('123') | integer(0)
// SORT_NUMERIC ==> boolean(true) | boolean(false) | string('123')
// SORT_STRING ==> boolean(true) | boolean(false) | string('true') | string('false') | string('0') | string('123') | string('AaA') | string('AAA')
// SORT_LOCALE_STRING ==> boolean(true) | boolean(false) | string('true') | string('false') | string('0') | string('123') | string('AaA') | string('AAA')

// PHP 8 -> 8.4
// SORT_REGULAR ==> boolean(true) | boolean(false)
// SORT_NUMERIC ==> boolean(true) | boolean(false) | string('123')
// SORT_STRING ==> boolean(true) | boolean(false) | string('true') | string('false') | string('0') | string('123') | string('AaA') | string('AAA')
// SORT_LOCALE_STRING ==> boolean(true) | boolean(false) | string('true') | string('false') | string('0') | string('123') | string('AaA') | string('AAA')

?>