Hello, and welcome to our analysis lesson. In this lesson, we will tackle a common problem in string manipulations with PHP. We will learn how to find all occurrences of a substring within a larger string. The techniques you will master today can be utilized in numerous situations, such as text processing and data analysis. Are you ready to get started? Let's jump right in!

Here is this unit's task: We have two arrays of strings, both of identical lengths — the first containing the "original" strings and the second containing the substrings. Our goal is to detect all occurrences of each substring within its corresponding original string and, finally, return an array that contains the starting indices of these occurrences. Remember, the index counting should start from 0.

Example

Let's consider the following arrays:

• Original Array: ["HelloWorld", "LearningPHP", "GoForBroke", "BackToBasics"]
• Substring Array: ["loW", "ear", "o", "Ba"].

The following are the expected outputs:

• In "HelloWorld", "loW" starts at index 3.
• In "LearningPHP", "ear" starts at index 1.
• In "GoForBroke", "o" appears at indices 1, 3, and 7.
• In "BackToBasics", "Ba" starts at indices 0 and 6.

Thus, when findSubString(["HelloWorld", "LearningPHP", "GoForBroke", "BackToBasics"], ["loW", "ear", "o", "Ba"]) is invoked, the function should return:

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1[
2    "The substring 'loW' was found in the original string 'HelloWorld' at position(s) 3.",
3    "The substring 'ear' was found in the original string 'LearningPHP' at position(s) 1.",
4    "The substring 'o' was found in the original string 'GoForBroke' at position(s) 1, 3, 7.",
5    "The substring 'Ba' was found in the original string 'BackToBasics' at position(s) 0, 6."
6]

Although this task may seem fairly straightforward, it can prove challenging. However, don't worry! We will break it down step by step.

Initially, we need to create a space to store our results. Can you think of a PHP data type that would be ideal for this task? That's right! An array would be perfect!

php
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1function findSubString($origStrs, $substrs) {
2    $result = [];

To pair original strings with their substrings, we use a for loop. In our case, both arrays share the same length, so we can use their indices to pair them correctly. To find the first occurrence of each substring in the corresponding original string, we utilize the strpos function:

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1    for ($i = 0; $i < count($origStrs); $i++) {
2        $start_pos = strpos($origStrs[$i], $substrs[$i]);

In strpos($haystack, $needle), we provide the substring that we intend to locate. The function starts its search from the beginning because we have not specified a starting position.

The next step is to find the subsequent instances of the substring in the original.

To do this, we will use a while loop. But when should we stop looking for more occurrences? When our strpos function returns false, it indicates there are no more matches to be found.

Each time we locate a match, we record its starting index in the $match_indices array, adjust the $start_pos, and begin the search anew:

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1        $match_indices = [];
2        while ($start_pos !== false) {
3            $match_indices[] = $start_pos;
4            $start_pos = strpos($origStrs[$i], $substrs[$i], $start_pos + strlen($substrs[$i]));
5        }

Finally, we utilize PHP's string concatenation to format the result for improved readability and add it to the $result array:

php
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1        if (!empty($match_indices)) {
2            $formatted_result = "The substring '" . $substrs[$i] . "' was found in the original string '" . $origStrs[$i] . "' at position(s) ";
3            $formatted_result .= implode(", ", $match_indices) . ".";
4            $result[] = $formatted_result;
5        }
6    }
7    return $result;
8}

That's it! We have completed the design of our function.

Here is the complete function, incorporating all the steps we have discussed so far:

php
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1<?php
2
3function findSubString($origStrs, $substrs) {
4    $result = [];
5
6    for ($i = 0; $i < count($origStrs); $i++) {
7        $start_pos = strpos($origStrs[$i], $substrs[$i]);
8        $match_indices = [];
9
10        while ($start_pos !== false) {
11            $match_indices[] = $start_pos;
12            $start_pos = strpos($origStrs[$i], $substrs[$i], $start_pos + strlen($substrs[$i]));
13        }
14
15        if (!empty($match_indices)) {
16            $formatted_result = "The substring '" . $substrs[$i] . "' was found in the original string '" . $origStrs[$i] . "' at position(s) ";
17            $formatted_result .= implode(", ", $match_indices) . ".";
18            $result[] = $formatted_result;
19        }
20    }
21
22    return $result;
23}
24
25// Call the function
26$result = findSubString(
27    ["HelloWorld", "LearningPHP", "GoForBroke", "BackToBasics"],
28    ["loW", "ear", "o", "Ba"]
29);
30
31foreach ($result as $res) {
32    echo $res . "\n";
33}

Well done! You've mastered a central operation in string manipulations in PHP — finding all occurrences of a substring in another string. Keep in mind that this algorithm has numerous applications in real-world scenarios. Now that we have intricately dissected the problem and provided a detailed solution, I encourage you to practice more. Future exercises will help you hone your skills further. Keep on coding and exploring!