Convert snake_case to camelCase in PHP and vice versa

cases_conversion.php

<?php

function snake_to_camel_case(string $string): string {
    $parts = explode('_', $string);

    $parts_count = count($parts);

    for ($i = 1; $i < $parts_count; ++$i) {
        $parts[$i] = ucfirst($parts[$i]);
    }

    return implode('', $parts);
}

function camel_to_snake_case(string $string): string {
    $s = preg_replace('/([a-z0-9])([A-Z])/', '$1_$2', $string);
    $s = preg_replace('/([A-Z])([A-Z])([a-z])/', '$1_$2$3', $s);

    return strtolower($s);
}

$tests = array(
    'simpleTest' => 'simple_test',
    'easy' => 'easy',
    'HTML' => 'html',
    'simpleXML' => 'simple_xml',
    'PDFLoad' => 'pdf_load',
    'startMIDDLELast' => 'start_middle_last',
    'AString' => 'a_string',
    'Some4Numbers234' => 'some4_numbers234',
    'TEST123String' => 'test123_string',
);
  
foreach ($tests as $test => $expected) {
    $result = camel_to_snake_case($test);

    if ($result === $expected) {
        echo "Pass: $test => $expected";
    } else {
        echo "Fail: $test => $expected [$result]";
    }

    echo '<br>';
}