原型

_mb_strlen( $str,  $encoding = null )

源文件

路径:wp-includes/compat.php

<?php
...
		| [xF1-xF3][x80-xBF]{3}
		| xF4[x80-x8F][x80-xBF]{2}
	)/x';
	// Start with 1 element instead of 0 since the first thing we do is pop.
	$chars = array( '' );
	do {
		// We had some string left over from the last round, but we counted it in that last round.
		array_pop( $chars );
		/*
		 * Split by UTF-8 character, limit to 1000 characters (last array element will contain
		 * the rest of the string).
		 */
		$pieces = preg_split( $regex, $str, 1000, PREG_SPLIT_DELIM_CAPTURE | PREG_SPLIT_NO_EMPTY );
		$chars = array_merge( $chars, $pieces );
	// If there's anything left over, repeat the loop.
	} while ( count( $pieces ) > 1 && $str = array_pop( $pieces ) );
	return join( '', array_slice( $chars, $start, $length ) );
}
if ( ! function_exists( 'mb_strlen' ) ) :
	/**
	 * Compat function to mimic mb_strlen().
	 *
	 * @ignore
	 * @since 4.2.0
	 *
	 * @see _mb_strlen()
	 *
	 * @param string      $str      The string to retrieve the character length from.
	 * @param string|null $encoding Optional. Character encoding to use. Default null.
	 * @return int String length of `$str`.
	 */
	function mb_strlen( $str, $encoding = null ) {
		return _mb_strlen( $str, $encoding );
	}
endif;
/**
 * Internal compat function to mimic mb_strlen().
...
?>

其他

英文文档:https://developer.wordpress.org/reference/functions/_mb_strlen/