String comparison functions in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the PHP function strcmp() do?

strcmp() compares two strings exactly and returns:

• 0 if both strings are equal
• A negative number if the first string is less than the second
• A positive number if the first string is greater than the second

beginner

How is strcasecmp() different from strcmp()?

strcasecmp() compares two strings ignoring case differences, while strcmp() is case-sensitive.

intermediate

What does strncmp() do in PHP?

strncmp() compares up to a specified number of characters from two strings. It returns 0 if the compared parts are equal.

beginner

Explain the return values of strcmp() with an example.

Example: strcmp('apple', 'banana') returns a negative number because 'apple' is less than 'banana' alphabetically.

Return values:

• 0 if equal
• <0 if first string < second string
• >0 if first string > second string

beginner

Why use strcasecmp() instead of strcmp()?

Use strcasecmp() when you want to compare strings without caring about uppercase or lowercase letters, like comparing user input where case should not matter.

What does strcmp('Hello', 'hello') return?

A 0

B A positive number

C A negative number

D Boolean true

strcmp() is case-sensitive. 'H' (uppercase) is less than 'h' (lowercase) in ASCII, so it returns a negative number.

Which function compares strings ignoring case?

A strcasecmp()

B strncmp()

C strcmp()

D strncasecmp()

strcasecmp() compares strings ignoring case differences.

What does strncmp('apple', 'apricot', 3) return?

A Boolean false

B Positive number

C Negative number

D 0

It compares first 3 characters: 'app' vs 'apr'. Since 'app' and 'apr' differ at the third character, 'p' vs 'r', 'p' is less than 'r', so it returns a negative number.

Which function compares only the first N characters of two strings?

A strncmp()

B strcmp()

C strcasecmp()

D str_replace()

strncmp() compares up to N characters of two strings.

If strcmp($a, $b) returns 0, what does it mean?

A $a is less than $b

B $a and $b are equal

C $a is greater than $b

D Comparison failed

A return value of 0 means both strings are exactly equal.

Describe how strcmp() works and what its return values mean.

Think about how two words are ordered in a dictionary.

When would you use strcasecmp() instead of strcmp()?

Consider comparing names or emails where uppercase and lowercase letters should not matter.

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