CamelCase to underscore with number support.

camelCaseToUnderscore.php

<?php

/**
 * @param      $input
 * @param bool $lowercase
 * @param int  $numbers 0 = No numbers, 1 = Numbers and 2 = Number with underscores
 *
 * @return mixed
 */
function camelCaseToUnderscore(
    $input,
    $lowercase = true,
    $numbers = self::NO_NUMBERS
) {
    $match = [
        "/([A-Z]+)/",
        "/_([A-Z]+)([A-Z][a-z])/",
    ];

    $replace = [
        "_$1",
        "_$1_$2",
    ];

    if (1 === $numbers) {
        $match = array_merge($match, [
            "/([0-9]+)/",
        ]);
        $replace = array_merge($replace, [
            "_$1",
        ]);
    } elseif (2 === $numbers) {
        $match = array_merge($match, [
            "/(.{1})$/",
            "/\d(?!$)/",
        ]);
        $replace = array_merge($replace, [
            "_$0",
            "_$0",
        ]);
    }

    $result = preg_replace($match, $replace, lcfirst($input));

    if ($lowercase) {
        $result = strtolower($result);
    }

    return $result;
}