String replace functions in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of str_replace with overlapping replacements

What is the output of the following PHP code?

PHP

<?php
$text = "abcabc";
$result = str_replace("abc", "x", $text);
echo $result;
?>

A xx

B abcabc

C xabc

D xxabc

💡 Hint

str_replace replaces all occurrences of the search string.

The function replaces every 'abc' with 'x'. Since 'abc' appears twice, the output is 'xx'.

❓ Predict Output

intermediate

Behavior of str_ireplace with case insensitive replacement

What will be the output of this PHP code?

PHP

<?php
$text = "Hello World!";
$result = str_ireplace("hello", "Hi", $text);
echo $result;
?>

A Hi hello World!

B hello World!

C Hello World!

D Hi World!

💡 Hint

str_ireplace ignores case when searching.

str_ireplace replaces 'hello' ignoring case, so 'Hello' becomes 'Hi'.

❓ Predict Output

advanced

Output of str_replace with array replacements

What is the output of this PHP code snippet?

PHP

<?php
$text = "I like cats and dogs.";
$result = str_replace(["cats", "dogs"], ["birds", "fish"], $text);
echo $result;
?>

A I like birds and dogs.

B I like birds and fish.

C I like cats and dogs.

D I like fish and birds.

💡 Hint

When arrays are used, replacements happen pairwise.

Each element in the search array is replaced by the corresponding element in the replacement array.

❓ Predict Output

advanced

Result of str_replace when search string is empty

What will this PHP code output?

PHP

<?php
$text = "hello";
$result = str_replace("", "x", $text);
echo $result;
?>

A xhelxloxlo

B hello

C xhello

D SyntaxError

💡 Hint

Replacing an empty string inserts the replacement between every character and at the start and end.

Replacing empty string inserts 'x' before, between, and after characters, resulting in 'xhelxloxlo'.

❓ Predict Output

expert

Output of nested str_replace calls

What is the output of this PHP code?

PHP

<?php
$text = "abc";
$result = str_replace("a", "b", str_replace("b", "c", $text));
echo $result;
?>

A acc

B ccc

C bcc

D bbc

💡 Hint

Inner str_replace runs first, then outer one.

Inner replaces 'b' with 'c' → 'acc'. Outer replaces 'a' with 'b' → 'bcc'.

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