php字符串截取 w3c

<?php
$string = "Hello, World!";
$start = 7;
$length = 5;

// 从起始位置截取到字符串末尾
$result1 = substr($string, $start);
echo $result1; // 输出: World!

// 从起始位置截取指定长度
$result2 = substr($string, $start, $length);
echo $result2; // 输出: World

// 从字符串末尾开始截取
$result3 = substr($string, -$start);
echo $result3; // 输出: World!

// 从字符串末尾开始截取指定长度
$result4 = substr($string, -$start, $length);
echo $result4; // 输出: World
?>

$string = "你好，世界！";
$string = mb_convert_encoding($string, 'UTF-8', 'auto');
$start = 1;
$length = 2;
$result = mb_substr($string, $start, $length, 'UTF-8');
echo $result; // 输出: 好，

$string = "Hello, World!";
$start = 10;
$length = 20;
$start = min($start, strlen($string));
$length = min($length, strlen($string) - $start);
$result = substr($string, $start, $length);
echo $result; // 输出: !