String comparison functions in PHP - Time & Space Complexity

Time Complexity: String comparison functions

O(n)

Understanding Time Complexity

When comparing two strings in PHP, it's important to know how the time taken grows as the strings get longer.

We want to understand how the comparison work costs change with string size.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$string1 = "hello world";
$string2 = "hello there";

if (strcmp($string1, $string2) === 0) {
    echo "Strings are equal.";
} else {
    echo "Strings are different.";
}
    

This code compares two strings character by character to check if they are exactly the same.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Comparing characters one by one from the start of both strings.
• How many times: Up to the length of the shorter string, or until a difference is found.

How Execution Grows With Input

As the strings get longer, the number of character comparisons can grow.

Input Size (n)	Approx. Operations
10	Up to 10 character comparisons
100	Up to 100 character comparisons
1000	Up to 1000 character comparisons

Pattern observation: The work grows roughly in direct proportion to the string length.

Final Time Complexity

Time Complexity: O(n)

This means the time to compare grows linearly with the length of the strings.

Common Mistake

[X] Wrong: "String comparison always takes the same time no matter the string length."

[OK] Correct: The function checks characters one by one, so longer strings usually take more time unless a difference is found early.

Interview Connect

Understanding how string comparison scales helps you reason about performance in real code, showing you think about efficiency clearly.

Self-Check

"What if we used a function that compares only the first few characters? How would the time complexity change?"

Practice

1. Which PHP function compares two strings in a case-sensitive way and returns 0 if they are exactly equal?

easy

A. strncmp

B. strcasecmp

C. strncasecmp

D. strcmp

Solution

1. Step 1: Understand case sensitivity in string comparison

   strcmp compares two strings exactly, including case.

2. Step 2: Identify function behavior

   strcmp returns 0 if strings are exactly equal, considering case.

3. Final Answer:

   strcmp -> Option D

4. Quick Check:

   Case-sensitive compare = strcmp [OK]

Hint: Case matters? Use strcmp or strncmp [OK]

Common Mistakes:

• Confusing strcmp with strcasecmp (case-insensitive)
• Using strncasecmp for full string compare
• Thinking strcmp ignores case

2. Which of the following is the correct syntax to compare first 3 characters of two strings ignoring case in PHP?

easy

A. strncasecmp($str1, $str2, 3)

B. strncmp($str1, $str2, 3)

C. strcasecmp($str1, $str2, 3)

D. strcmp($str1, $str2, 3)

Solution

1. Step 1: Identify function for case-insensitive partial compare

   strncasecmp compares first N characters ignoring case.

2. Step 2: Check syntax correctness

   strncasecmp($str1, $str2, 3) correctly compares first 3 chars case-insensitively.

3. Final Answer:

   strncasecmp($str1, $str2, 3) -> Option A

4. Quick Check:

   Partial case-insensitive compare = strncasecmp [OK]

Hint: Partial + ignore case = strncasecmp [OK]

Common Mistakes:

• Using strncmp which is case-sensitive
• Passing length to strcasecmp (no length param)
• Using strcmp with length parameter

3. What will be the output of the following PHP code?

$result = strcasecmp('Hello', 'hello');
echo $result;

medium

A. -1

B. 0

C. 1

D. Error

Solution

1. Step 1: Understand strcasecmp behavior

   It compares strings ignoring case and returns 0 if equal.

2. Step 2: Compare 'Hello' and 'hello'

   Ignoring case, both strings are equal, so result is 0.

3. Final Answer:

   0 -> Option B

4. Quick Check:

   strcasecmp('Hello', 'hello') = 0 [OK]

Hint: strcasecmp returns 0 if strings equal ignoring case [OK]

Common Mistakes:

• Expecting 1 or -1 for equal strings
• Confusing case-sensitive strcmp with strcasecmp
• Thinking it throws error for case differences

4. Identify the error in this PHP code snippet:

if (strncmp('apple', 'Apple', 5) == 0) {
    echo 'Match';
} else {
    echo 'No match';
}

medium

A. No error, code works correctly

B. Use strcasecmp instead of strncmp for case-insensitive compare

C. Use strncasecmp instead of strncmp for case-insensitive compare

D. Change 5 to length of shorter string

Solution

1. Step 1: Analyze strncmp behavior

   strncmp is case-sensitive, so 'apple' != 'Apple'.

2. Step 2: Fix for case-insensitive compare

   Use strncasecmp to ignore case in first 5 chars.

3. Final Answer:

   Use strncasecmp instead of strncmp for case-insensitive compare -> Option C

4. Quick Check:

   Case-insensitive partial compare needs strncasecmp [OK]

Hint: strncmp is case-sensitive; use strncasecmp to ignore case [OK]

Common Mistakes:

• Assuming strncmp ignores case
• Using strcasecmp with length parameter
• Changing length unnecessarily

5. You want to sort an array of strings alphabetically ignoring case in PHP. Which function is best to use as a comparison callback for usort?

hard

A. strcasecmp

B. strcmp

C. strncmp

D. strncasecmp

Solution

1. Step 1: Understand sorting with case-insensitivity

   Sorting ignoring case requires a case-insensitive comparison function.

2. Step 2: Choose correct function for full string compare ignoring case

   strcasecmp compares full strings ignoring case, ideal for sorting.

3. Final Answer:

   strcasecmp -> Option A

4. Quick Check:

   Case-insensitive full compare for sorting = strcasecmp [OK]

Hint: Sort ignoring case? Use strcasecmp as callback [OK]

Common Mistakes:

• Using strcmp which is case-sensitive
• Using strncmp or strncasecmp which compare partial strings
• Passing no callback to usort