Question

_mb_substr( $str,  $start,  $length = null,  $encoding = null )

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description

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源代码

File: wp-includes/compat.php

	 * @param int|null    $length   Optional. Maximum number of characters to extract from `$str`.
	 *                              Default null.
	 * @param string|null $encoding Optional. Character encoding to use. Default null.
	 * @return string Extracted substring.
	 */
	function mb_substr( $str, $start, $length = null, $encoding = null ) {
		return _mb_substr( $str, $start, $length, $encoding );
	}
endif;

/**
 * Internal compat function to mimic mb_substr().
 *
 * Only understands UTF-8 and 8bit.  All other character sets will be treated as 8bit.
 * For $encoding === UTF-8, the $str input is expected to be a valid UTF-8 byte sequence.
 * The behavior of this function for invalid inputs is undefined.
 *
 * @ignore
 * @since 3.2.0
 *
 * @param string      $str      The string to extract the substring from.
 * @param int         $start    Position to being extraction from in `$str`.
 * @param int|null    $length   Optional. Maximum number of characters to extract from `$str`.
 *                              Default null.
 * @param string|null $encoding Optional. Character encoding to use. Default null.
 * @return string Extracted substring.
 */
function _mb_substr( $str, $start, $length = null, $encoding = null ) {
	if ( null === $encoding ) {
		$encoding = get_option( 'blog_charset' );
	}

	/*
	 * The solution below works only for UTF-8, so in case of a different
	 * charset just use built-in substr().
	 */
	if ( ! in_array( $encoding, array( 'utf8', 'utf-8', 'UTF8', 'UTF-8' ) ) ) {
		return is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );
	}

	if ( _wp_can_use_pcre_u() ) {
		// Use the regex unicode support to separate the UTF-8 characters into an array.
		preg_match_all( '/./us', $str, $match );

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