strspn() 函數在 PHP 中主要用于獲取字符串中特定字符的起始位置。它接受兩個參數：第一個參數是要檢查的字符串，第二個參數是要查找的字符集。strspn() 函數會返回在字符串開頭連續匹配給定字符集的字符數。

以下是 strspn() 函數的一些使用場景：

1. 檢查字符串是否以特定子字符串開頭：

$str = "Hello, world!";
$subStr = "Hello";
$result = strspn($str, $subStr);
if ($result > 0) {
    echo "The string starts with the given substring.";
} else {
    echo "The string does not start with the given substring.";
}

2. 提取字符串中的特定部分：

$str = "The quick brown fox jumps over the lazy dog";
$words = str_split($str);
$result = strspn($words[0], "a-zA-Z");
echo "The first word has " . $result . " alphabetic characters.";

3. 過濾不需要的字符：

$str = "Hello, world! 123";
$allowedChars = "a-zA-Z ";
$result = strspn($str, $allowedChars);
$filteredStr = substr($str, $result);
echo "Filtered string: " . $filteredStr;

4. 字符串替換：

$str = "The quick brown fox jumps over the lazy dog";
$searchChars = "aeiouAEIOU";
$replaceChars = "";
$result = strspn($str, $searchChars);
$replacedStr = substr_replace($str, $replaceChars, $result, strlen($searchChars));
echo "Replaced string: " . $replacedStr;

總之，strspn() 函數在處理字符串時非常有用，特別是在需要檢查字符串開頭或提取特定字符集的情況下。