substr在php

substr(string, start, length)

$string = "Hello, world!";
$start = -3;
$length = 2;

// 确保 start 和 length 在合理范围内
if ($start < 0) {
    $start += strlen($string);
}
if ($length < 0) {
    $length = 0;
}

$result = substr($string, $start, $length);
echo $result; // 输出 "rl"

$string = "Hello, world!";
$start = 100;

// 检查 start 是否超出字符串长度
if ($start >= strlen($string)) {
    echo "Start position is out of range.";
} else {
    $result = substr($string, $start);
    echo $result;
}

$string = "";
$start = 0;
$length = 5;

// 检查字符串是否为空
if (empty($string)) {
    echo "The input string is empty.";
} else {
    $result = substr($string, $start, $length);
    echo $result;
}