In PHP, what is the functionality of strstr() and how does stristr() differ from it?

Introduction / Context:
String search operations are common in PHP, and the strstr() and stristr() functions provide simple ways to find substrings. While they seem similar, they differ in how they treat letter case. Interview questions often ask about these functions to verify that a candidate understands case sensitivity in string operations and can choose the appropriate function for different scenarios.

Given Data / Assumptions:

• We are working with plain PHP strings.
• strstr() and stristr() both take a haystack string and a needle substring.
• The functions return either a portion of the string or false on failure.
• The main difference between them is case sensitivity.

Concept / Approach:
strstr() searches for the first occurrence of a substring within a string, respecting case, and returns the part of the string from that position to the end. If the substring is not found, it returns false. stristr() has the same basic behaviour but ignores case differences between the haystack and the needle, making it useful when you want to find a substring without worrying about whether it is written in upper or lower case. Both functions can optionally return the part before the match when the third parameter is used in newer PHP versions.

Step-by-Step Solution:
Step 1: For example, strstr("Hello World", "World") returns "World" because it finds the substring World exactly as written and returns from that position to the end of the string. Step 2: If you call strstr("Hello World", "world"), the function returns false because the lowercase w does not match the uppercase W when case sensitivity is enforced. Step 3: Using stristr("Hello World", "world") instead returns "World" because stristr() performs a case insensitive search and treats World and world as equivalent. Step 4: In both cases, the primary behaviour is to locate the first occurrence of the needle and return the rest of the string starting from that position, or false if no match is found. Step 5: Some code uses these functions to test whether a substring exists by comparing the return value to false, while others may use the returned string for further processing. Step 6: From this discussion, it is clear that the main difference is case sensitivity, matching the description in option a.

Verification / Alternative check:
You can verify this behaviour by running short PHP scripts that call strstr() and stristr() with various combinations of upper and lower case strings. The outputs will show that strstr() fails when case does not match, while stristr() still finds the substring. Additional experiments with the optional before needle parameter in newer PHP versions will show that both functions share the same extended capabilities beyond simple case handling.

Why Other Options Are Wrong:
Option b is wrong because neither function reverses or sorts strings; they only search for substrings. Option c is incorrect because both functions operate on strings, not arrays or purely numeric values as separate types. Option d is wrong because strstr() and stristr() are string search functions, not hashing functions, and have nothing to do with md5() or sha1().

Common Pitfalls:
Common pitfalls include forgetting to check for false explicitly and instead using the returned string in a context where an empty string or false has different meaning. Another issue is overlooking the performance cost of repeated substring searches on large strings. Developers should choose stristr() only when case insensitive matching is necessary and prefer strstr() when exact matching is required. Understanding the difference helps avoid subtle bugs when matching user input or processing text data.

Final Answer:
strstr() and stristr() both search a string for a substring and return the part of the string from the first match onward, but strstr() is case sensitive whereas stristr() performs a case insensitive search.