String replace functions in PHP - Time & Space Complexity

Time Complexity: String replace functions

O(n)

Understanding Time Complexity

When we use string replace functions, we want to know how the time it takes changes as the string gets longer.

We ask: How does the work grow when the input string grows?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

$string = "hello world hello";
$search = "hello";
$replace = "hi";

$result = str_replace($search, $replace, $string);
    

This code replaces all occurrences of "hello" with "hi" in the given string.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Scanning the entire string to find matches of the search word.
• How many times: The function checks each character in the string once or more to find all matches.

How Execution Grows With Input

As the string gets longer, the function needs to look at more characters to find matches.

Input Size (n)	Approx. Operations
10	About 10 checks
100	About 100 checks
1000	About 1000 checks

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 replace grows linearly as the string gets longer.

Common Mistake

[X] Wrong: "Replacing a word in a string always takes the same time no matter how long the string is."

[OK] Correct: The function must check the whole string to find matches, so longer strings take more time.

Interview Connect

Understanding how string operations scale helps you write efficient code and explain your choices clearly in interviews.

Self-Check

"What if we replaced multiple different words at once? How would the time complexity change?"

Practice

1. Which PHP function is used to replace all occurrences of a string with another string?

easy

A. strpos()

B. str_replace()

C. substr()

D. strlen()

Solution

1. Step 1: Understand the purpose of str_replace()

   The function str_replace() replaces all occurrences of a search string with a replacement string in a given text.

2. Step 2: Compare with other functions

   substr() extracts part of a string, strpos() finds position of a substring, strlen() returns string length. None replace text.

3. Final Answer:

   str_replace() -> Option B

4. Quick Check:

   Replace text = str_replace() [OK]

Hint: Remember: str_replace replaces text, others do different tasks [OK]

Common Mistakes:

• Confusing str_replace with substr
• Using strpos instead of replace
• Expecting strlen to replace text

2. Which of the following is the correct syntax to replace 'cat' with 'dog' in the string $text using PHP?

easy

A. str_replace('dog', 'cat', $text);

B. replace('cat', 'dog', $text);

C. str_replace($text, 'cat', 'dog');

D. str_replace('cat', 'dog', $text);

Solution

1. Step 1: Check the correct parameter order of str_replace()

   The correct order is str_replace(search, replace, subject). So 'cat' is search, 'dog' is replace, $text is subject.

2. Step 2: Validate each option

   str_replace('cat', 'dog', $text); matches correct order. replace('cat', 'dog', $text); uses a non-existent function. str_replace($text, 'cat', 'dog'); swaps parameters incorrectly. str_replace('dog', 'cat', $text); swaps search and replace.

3. Final Answer:

   str_replace('cat', 'dog', $text); -> Option D

4. Quick Check:

   str_replace(search, replace, subject) [OK]

Hint: Remember parameter order: search, replace, then subject [OK]

Common Mistakes:

• Swapping search and replace
• Using wrong function name
• Incorrect parameter order

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

$text = "I love apples.";
$result = str_replace("apples", "oranges", $text);
echo $result;

medium

A. I love oranges.

B. I love apples.

C. I love apples.oranges

D. Syntax error

Solution

1. Step 1: Understand str_replace behavior

   str_replace replaces all occurrences of the search string with the replacement string in the given text.

2. Step 2: Apply replacement to the string

   "apples" is replaced by "oranges" in "I love apples.", resulting in "I love oranges.".

3. Final Answer:

   I love oranges. -> Option A

4. Quick Check:

   Replace 'apples' with 'oranges' = I love oranges. [OK]

Hint: str_replace returns new string; echo the result to see changes [OK]

Common Mistakes:

• Expecting original string unchanged
• Not assigning result to variable
• Syntax errors in code

4. Identify the error in the following PHP code snippet:

$text = "Hello world!";
str_replace('world', 'PHP', $text);
echo $text;

medium

A. Missing third argument in str_replace

B. Syntax error in str_replace call

C. str_replace does not modify $text directly

D. echo statement is incorrect

Solution

1. Step 1: Check str_replace usage

   The str_replace call has the correct three arguments: search='world', replace='PHP', subject=$text.

2. Step 2: Understand string immutability in PHP

   str_replace returns a new string; it does not change the original string $text unless assigned back, e.g., $text = str_replace('world', 'PHP', $text);

3. Final Answer:

   str_replace does not modify $text directly -> Option C

4. Quick Check:

   Assign result to variable to update string [OK]

Hint: Assign str_replace result to variable; original string stays same [OK]

Common Mistakes:

• Not assigning result back to variable
• Assuming str_replace changes string in place

5. You want to replace multiple words in a string at once: replace 'red' with 'blue' and 'green' with 'yellow' in $text. Which code correctly does this in PHP?

hard

A. $result = str_replace(['red', 'green'], ['blue', 'yellow'], $text);

B. $result = str_replace('red', 'blue', $text);
str_replace('green', 'yellow', $text);

C. $result = str_replace('red', 'blue', $text) && str_replace('green', 'yellow', $text);

D. $result = str_replace('red' => 'blue', 'green' => 'yellow', $text);

Solution

1. Step 1: Understand str_replace with arrays

   str_replace accepts arrays for search and replace to perform multiple replacements in one call.

2. Step 2: Analyze each option

   $result = str_replace(['red', 'green'], ['blue', 'yellow'], $text); uses arrays correctly. $result = str_replace('red', 'blue', $text); str_replace('green', 'yellow', $text); performs first replacement but second call discards result and uses original $text, so 'green' remains. $result = str_replace('red', 'blue', $text) && str_replace('green', 'yellow', $text); uses && which returns boolean, not string. $result = str_replace('red' => 'blue', 'green' => 'yellow', $text); uses invalid syntax.

3. Final Answer:

   $result = str_replace(['red', 'green'], ['blue', 'yellow'], $text); -> Option A

4. Quick Check:

   Use arrays in str_replace for multiple replacements [OK]

Hint: Use arrays in str_replace to replace many words at once [OK]

Common Mistakes:

• Using multiple str_replace without chaining
• Using logical operators instead of chaining
• Wrong syntax for arrays in str_replace