How is mb_substr used in PHP?

The mb_substr function in PHP is used for extracting substrings and can handle multi-byte characters. Its usage is as follows:

mb_substr(string $string, int $start, [int $length = NULL [, string $encoding = mb_internal_encoding()]]): string|false

Explanation of parameters:

1. string: the string that needs to be trimmed.
2. Start: The starting position of the substring. Positive numbers indicate counting from the left side, while negative numbers indicate counting from the right side.
3. Length (optional): The number of characters to be extracted. A positive number indicates the number of characters to be extracted from the left, while a negative number indicates the number of characters to be extracted from the right.
4. Encoding (optional): Specify the character encoding to be used, defaulting to the encoding returned by the mb_internal_encoding() function.

Return value:

1. When the interception is successful, the intercepted string will be returned.
2. If the interception fails or encounters an error, return false.

原文：我们需要合适的资源来完成这项工作。

重述：We need the right resources to complete this task.

$str = '你好，世界！';
echo mb_substr($str, 0, 2);  // 输出：你好
echo mb_substr($str, 3, 2);  // 输出：世界
echo mb_substr($str, -2);    // 输出：界！

Please note that the mb_substr function requires support from the mbstring extension. Make sure to check if this extension is enabled before using it.