如何在 PHP 中使用遞歸來定位字串的第二次（或第 n 次）出現？

Locating Multiple Occurrences of a String with strpos: Unveiling the Second Occurrence

strpos is a powerful function that allows programmers to determine the position of the first occurrence of a substring within a string. However, what if the objective is to identify the second occurrence? This question often arises, particularly when dealing with complex data manipulation tasks.

Answer: Embracing Recursion for Successive Searches

To address this challenge, developers can leverage recursion, a technique particularly suited for this scenario. Here's how it's achieved:

1. First Occurrence Retrieval: Begin by using strpos to find the position of the first occurrence of the substring.
2. Recursive Call: If the desired occurrence is greater than 1, perform a recursive call to strpos, this time specifying a starting position that is shifted forward by the length of the substring obtained from the first occurrence.
3. Recurse Until the Desired Occurrence: Repeat步骤2 until the desired occurrence is found.

Custom Function to Simplify the Process

To streamline this process, a custom function can be developed:

function strposX($haystack, $needle, $number) {
    if ($number == 1) {
        return strpos($haystack, $needle);
    } elseif ($number > 1) {
        return strpos($haystack, $needle, strposX($haystack, $needle, $number - 1) + strlen($needle));
    } else {
        return error_log('Error: Value for parameter $number is out of range');
    }
}

Alternatively, a simplified version can be utilized:

function strposX($haystack, $needle, $number = 0)
{
    return strpos($haystack, $needle,
        $number > 1 ?
        strposX($haystack, $needle, $number - 1) + strlen($needle) : 0
    );
}

By incorporating these approaches, programmers can effectively identify multiple occurrences of a substring, including the second occurrence, empowering them with enhanced string manipulation capabilities.

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