Welcome to our focused exploration of PHP's array functions and their powerful applications in algorithmic challenges. In this lesson, "Deciphering Uniqueness and Anagram Mysteries with PHP Arrays," we'll delve into how these functions can be harnessed to efficiently tackle problems commonly seen in technical interviews.

Imagine: you’re presented with an extensive list of words, with the task to pinpoint the final word that stands alone — the last non-repeating word. This mirrors the challenge of sorting through a database of unique identifiers to identify a distinct one near the end.

A straightforward solution iterates in reverse through each word, comparing it with every other word to check for uniqueness. This brute-force approach results in a time complexity of $O(n^2)$, making it impractical for large datasets, where $n$ is the number of words.

We can capitalize on associative arrays in PHP to efficiently count word occurrences and identify uniqueness:

1. Initialize an associative array for word counts:

   php
   Copy to clipboard
   1$wordCounts = [];

2. Count each word's occurrences as you traverse the list:

   php
   Copy to clipboard
   1foreach ($words as $word) {
   2    if (isset($wordCounts[$word])) {
   3        $wordCounts[$word]++;
   4    } else {
   5        $wordCounts[$word] = 1;
   6    }
   7}

3. Identify the last unique word by traversing the array from the end:

   php
   Copy to clipboard
   1$lastUniqueWord = "";
   2for ($i = count($words) - 1; $i >= 0; $i--) {
   3    if ($wordCounts[$words[$i]] == 1) {
   4        $lastUniqueWord = $words[$i];
   5        break;
   6    }
   7}

4. Return the unique word:

   php
   Copy to clipboard
   1return $lastUniqueWord;

Here is the full code!

php
Copy to clipboard
1<?php
2function findLastUniqueWord($words) {
3    // Step 1: Initialize an associative array for word counts
4    $wordCounts = [];
5
6    // Step 2: Count each word's occurrences as you traverse the list
7    foreach ($words as $word) {
8        if (isset($wordCounts[$word])) {
9            $wordCounts[$word]++;
10        } else {
11            $wordCounts[$word] = 1;
12        }
13    }
14
15    // Step 3: Identify the last unique word by traversing the array from the end
16    $lastUniqueWord = "";
17    for ($i = count($words) - 1; $i >= 0; $i--) {
18        if ($wordCounts[$words[$i]] == 1) {
19            $lastUniqueWord = $words[$i];
20            break;
21        }
22    }
23
24    // Step 4: Return the unique word
25    return $lastUniqueWord;
26}
27
28// Example usage
29$words = ["apple", "banana", "apple", "orange", "banana", "kiwi"];
30echo findLastUniqueWord($words); // Output: kiwi
31?>

This approach accomplishes a time complexity of $O(n)$. Iterating through the list twice (once for counting and once for finding uniqueness) is linear, as each operation on an associative array (insert or lookup) generally has an average time complexity of $O(1)$.

Next, consider a scenario with two arrays of strings, aiming to identify all words in the first array with an anagram present in the second array.

To efficiently find anagrams, we'll assign a sorted character signature to each word, leveraging PHP arrays for efficient lookups:

1. Create a function to produce sorted character signatures:

   php
   Copy to clipboard
   1function sortCharacters($input) {
   2    $characters = str_split($input);
   3    sort($characters);
   4    return implode('', $characters);
   5}

2. Store signatures from array2 in an associative array for quick comparison:

   php
   Copy to clipboard
   1$sortedWordsInArray2 = [];
   2foreach ($array2 as $word) {
   3    $sortedWordsInArray2[sortCharacters($word)] = true;
   4}

3. Find anagrams by comparing each word's signature in array1 with stored signatures:

   php
   Copy to clipboard
   1$result = [];
   2foreach ($array1 as $word) {
   3    $sortedWord = sortCharacters($word);
   4    if (isset($sortedWordsInArray2[$sortedWord])) {
   5        $result[] = $word;
   6    }
   7}

4. Return the list of anagrams:

   php
   Copy to clipboard
   1return $result;

Here is the full code!

php
Copy to clipboard
1<?php
2function sortCharacters($input) {
3    $characters = str_split($input);
4    sort($characters);
5    return implode('', $characters);
6}
7
8function findAnagrams($array1, $array2) {
9    // Step 2: Store signatures from array2 in an associative array for quick comparison
10    $sortedWordsInArray2 = [];
11    foreach ($array2 as $word) {
12        $sortedWordsInArray2[sortCharacters($word)] = true;
13    }
14
15    // Step 3: Find anagrams by comparing each word's signature in array1 with stored signatures
16    $result = [];
17    foreach ($array1 as $word) {
18        $sortedWord = sortCharacters($word);
19        if (isset($sortedWordsInArray2[$sortedWord])) {
20            $result[] = $word;
21        }
22    }
23
24    // Step 4: Return the list of anagrams
25    return $result;
26}
27
28// Example usage
29$array1 = ["listen", "silent", "enlist", "google"];
30$array2 = ["inlets", "banana", "google"];
31
32print_r(findAnagrams($array1, $array2)); // Output: Array ( [0] => listen [1] => silent [2] => enlist [3] => google )
33?>

This method achieves significant efficiency through the combination of character sorting and associative array lookups. The total complexity is dominated by the sorting operation, which operates in $O(m \log m)$ for each word (where $m$ is the average length of the words). Hence, the overall complexity is $O(n \cdot m \log m + k \cdot m \log m)$, with $n$ and $k$ being the numbers of words in array1 and array2, respectively.

Throughout this lesson, we harnessed PHP's array functions to solve the "Unique Echo" and "Anagram Matcher" challenges efficiently. These techniques underscore the importance of using PHP's array operations to manage complexity, a skill revered in technical interviews. Next, you’ll engage in hands-on exercises to deepen your understanding and refinement of these computational techniques.