Run Text Analysis with loophp/collection.

TextAnalysis.php

<?php

declare(strict_types=1);

include __DIR__ . '/vendor/autoload.php';

use loophp\collection\Collection;

$collection = Collection::with(file_get_contents('http://loripsum.net/api'))
    // Filter out some characters.
    ->filter(
        static function ($item, $key): bool {
            return (bool) preg_match('/^[a-zA-Z]+$/', $item);
        }
    )
    // Lowercase each character.
    ->map(static function (string $letter): string {
        return mb_strtolower($letter);
    })
    // Run the frequency tool.
    ->frequency()
    // Flip keys and values.
    ->flip()
    // Sort values.
    ->sort()
    // Convert to array.
    ->all();

print_r($collection);

/** 
Array
(
    [x] => 6
    [h] => 9
    [g] => 10
    [f] => 13
    [b] => 17
    [q] => 27
    [v] => 32
    [l] => 48
    [d] => 51
    [p] => 53
    [c] => 58
    [m] => 92
    [o] => 94
    [n] => 102
    [r] => 103
    [s] => 105
    [u] => 124
    [a] => 138
    [t] => 150
    [i] => 180
    [e] => 189
)
*/