An `mb_substr_replace()` implementation

mb_substr_replace.php

<?php

/**
 * Replace text within a portion of a multi-byte string
 * <p>
 * Performs a multi-byte safe <i>{@link substr_replace()}</i> operation replacing a copy of <i>string</i> delimited by
 * the <i>start</i> and (optionally) <i>length</i> parameters with the string given in <i>replacement</i>.
 *
 * @see http://php.net/manual/en/function.substr-replace.php
 * @see http://php.net/manual/en/function.mb-substr.php
 *
 * @param mixed $string The input string.
 * <p>
 * An array of strings can be provided, in which case the replacements will occur on each string in turn. In this case,
 * the <i>replacement</i>, <i>start</i>, <i>length</i> and <i>encoding</i> parameters may be provided either as scalar
 * values to be applied to each input string in turn, or as arrays, in which case the corresponding array element will
 * be used for each input string.
 *
 * @param mixed $replacement The replacement string.
 *
 * @param mixed $start If <i>start</i> is positive, the replacing will begin at the <i>start</i>'th offset into
 * <i>string</i>.
 * <p>
 * If <i>start</i> is negative, the replacing will begin at the <i>start</i>'th character from the end of <i>string</i>.
 *
 * @param mixed $length [optional]
 * <p>
 * If given and is positive, it represents the length of the portion of <i>string</i> which is to be replaced. If it is
 * negative, it represents the number of characters from the end of <i>string</i> at which to stop replacing. If it is
 * not given or equals to <b>NULL</b> or an empty string, then it will default to strlen( <i>string</i> ); i.e. end the
 * replacing at the end of <i>string</i>. If <i>length</i> is zero then this function will have the effect of inserting
 * <i>replacement</i> into <i>string</i> at the given <i>start</i> offset.
 *
 * @param mixed $encoding [optional]
 * <p>
 * The <i>encoding</i> parameter is the character encoding. If it is omitted, the internal character encoding value will
 * be used.
 *
 * @return mixed The result string is returned. If <i>string</i> is an array then array is returned.
 */
function mb_substr_replace($string, $replacement, $start, $length = null, $encoding = null)
{
    if (!$encoding) {
        $encoding = mb_internal_encoding();
    }

    if (is_array($string)) {
        $stringCount = count($string);

        if (is_array($replacement)) {
            if (count($replacement) < $stringCount) {
                $replacement = array_pad($replacement, $stringCount, '');
            }
        } else {
            $replacement = array_fill(0, $stringCount, $replacement);
        }

        if (is_array($start)) {
            if (count($start) < $stringCount) {
                $start = array_pad($start, $stringCount, 0);
            }
        } else {
            $start = array_fill(0, $stringCount, $start);
        }

        if (is_array($length)) {
            if (count($length) < $stringCount) {
                $length = array_pad($length, $stringCount, null);
            }
        } else {
            $length = array_fill(0, $stringCount, $length);
        }

        if (is_array($encoding)) {
            if (count($encoding) < $stringCount) {
                $encoding = array_pad($encoding, $stringCount, mb_internal_encoding());
            }
        } else {
            $encoding = array_fill(0, $stringCount, $encoding);
        }

        return array_map(__METHOD__, $string, $replacement, $start, $length, $encoding);
    }

    $stringLength = mb_strlen($string, $encoding);

    if ($start < 0) {
        if (-$start < $stringLength) {
            $startNormalized = $stringLength + $start;
        } else {
            $startNormalized = 0;
        }
    } else if ($start > $stringLength) {
        $startNormalized = $stringLength;
    } else {
        $startNormalized = $start;
    }

    if ($length === null || $length === '') {
        $start2 = $stringLength;
    } else if ($length < 0) {
        $start2 = $stringLength + $length;
        if ($start2 < $startNormalized) {
            $start2 = $startNormalized;
        }
    } else {
        $start2 = $startNormalized + $length;
    }

    $leader = $startNormalized
        ? mb_substr($string, 0, $startNormalized, $encoding)
        : '';

    $trailer = $start2 < $stringLength
        ? mb_substr($string, $start2, null, $encoding)
        : '';

    return "{$leader}{$replacement}{$trailer}";
}