Common validation patterns in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this PHP code validating an email?

Consider the following PHP code that checks if an email is valid using filter_var. What will it output?

PHP

<?php
$email = '[e-mail]';
if (filter_var($email, FILTER_VALIDATE_EMAIL)) {
    echo 'Valid email';
} else {
    echo 'Invalid email';
}
?>

A Valid email

B Invalid email

C Syntax error

D No output

💡 Hint

filter_var with FILTER_VALIDATE_EMAIL returns the email if valid, false otherwise.

The email '[e-mail]' is a valid email format, so filter_var returns the email string, making the if condition true and printing 'Valid email'.

❓ Predict Output

intermediate

What does this PHP code output when validating a numeric string?

Given this PHP snippet that uses ctype_digit to check if a string contains only digits, what will it output?

PHP

<?php
$input = '12345';
if (ctype_digit($input)) {
    echo 'All digits';
} else {
    echo 'Not all digits';
}
?>

A All digits

B Not all digits

C Warning: ctype_digit expects string

D No output

💡 Hint

ctype_digit returns true if all characters are digits.

The string '12345' contains only digits, so ctype_digit returns true and the code prints 'All digits'.

🔧 Debug

advanced

What error does this PHP code raise when validating a URL?

Examine this PHP code that attempts to validate a URL. What error or output will it produce?

PHP

<?php
$url = 'http://example.com';
if (filter_var($url, FILTER_VALIDATE_URL)) {
    echo 'Valid URL';
} else {
    echo 'Invalid URL';
}
?>

A Warning: filter_var expects parameter 2 to be int

B Invalid URL

C Valid URL

D Syntax error

💡 Hint

FILTER_VALIDATE_URL checks if the string is a valid URL format.

The string 'http://example.com' is a valid URL, so filter_var returns the URL string, making the if condition true and printing 'Valid URL'.

❓ Predict Output

advanced

What is the output of this PHP code validating a password length?

This PHP code checks if a password length is at least 8 characters. What will it output?

PHP

<?php
$password = 'pass123';
if (strlen($password) >= 8) {
    echo 'Password is strong';
} else {
    echo 'Password too short';
}
?>

A No output

B Password is strong

C Warning: strlen expects parameter 1 to be string

D Password too short

💡 Hint

strlen returns the length of the string.

The password 'pass123' has 7 characters, which is less than 8, so the code prints 'Password too short'.

🧠 Conceptual

expert

Which option correctly describes the behavior of PHP's filter_var with FILTER_VALIDATE_INT and options?

Consider the following PHP code snippet:

$options = ['options' => ['min_range' => 1, 'max_range' => 10]];
$value = 5;
$result = filter_var($value, FILTER_VALIDATE_INT, $options);

What does $result contain after execution?

A The string '5' because filter_var always returns strings

B The integer 5 because it is within the range 1 to 10

C Null because the options array is ignored

D Boolean false because filter_var does not accept options with FILTER_VALIDATE_INT

💡 Hint

FILTER_VALIDATE_INT accepts an options array to specify min and max range.

filter_var returns the integer value if it is within the specified range. Since 5 is between 1 and 10, $result is 5 (int).

Practice

1.

Which PHP function is commonly used to validate if a string is a valid email address?

easy

A. preg_match with '/^\d+$/'

B. filter_var with FILTER_VALIDATE_EMAIL

C. strlen to check length

D. strtolower to convert to lowercase

Solution

1. Step 1: Understand email validation in PHP

   PHP provides a built-in filter called FILTER_VALIDATE_EMAIL to check if a string is a valid email.

2. Step 2: Identify the correct function usage

   filter_var with FILTER_VALIDATE_EMAIL returns the email if valid, or false if invalid.

3. Final Answer:

   filter_var with FILTER_VALIDATE_EMAIL -> Option B

4. Quick Check:

   Email validation = filter_var with FILTER_VALIDATE_EMAIL [OK]

Hint: Use filter_var with FILTER_VALIDATE_EMAIL for email checks [OK]

Common Mistakes:

• Using preg_match with wrong regex for email
• Checking length instead of format
• Using string functions unrelated to validation

2.

Which of the following is the correct syntax to validate a URL using filter_var in PHP?

?

easy

A. filter_var($url, FILTER_VALIDATE_INT);

B. filter_var($url, FILTER_VALIDATE_EMAIL);

C. filter_var($url, FILTER_VALIDATE_URL);

D. filter_var($url, FILTER_VALIDATE_BOOLEAN);

Solution

1. Step 1: Identify the filter for URL validation

   FILTER_VALIDATE_URL is the correct filter to check if a string is a valid URL.

2. Step 2: Confirm correct function call

   filter_var takes the variable and the filter constant as parameters.

3. Final Answer:

   filter_var($url, FILTER_VALIDATE_URL); -> Option C

4. Quick Check:

   URL validation = filter_var with FILTER_VALIDATE_URL [OK]

Hint: Use FILTER_VALIDATE_URL with filter_var for URL validation [OK]

Common Mistakes:

• Using FILTER_VALIDATE_EMAIL for URLs
• Using integer or boolean filters incorrectly
• Missing the filter parameter

3.

What will be the output of the following PHP code?

$phone = '123-456-7890';
if (preg_match('/^\d{3}-\d{3}-\d{4}$/', $phone)) {
    echo 'Valid phone';
} else {
    echo 'Invalid phone';
}

medium

A. Syntax error

B. Invalid phone

C. No output

D. Valid phone

Solution

1. Step 1: Analyze the regex pattern

   The pattern /^\d{3}-\d{3}-\d{4}$/ matches exactly 3 digits, a dash, 3 digits, a dash, then 4 digits.

2. Step 2: Check the input string against the pattern

   The string '123-456-7890' matches this pattern exactly.

3. Final Answer:

   Valid phone -> Option D

4. Quick Check:

   Regex matches phone format = Valid phone [OK]

Hint: Match exact pattern with preg_match regex anchors ^ and $ [OK]

Common Mistakes:

• Forgetting anchors ^ and $ in regex
• Using wrong quantifiers in regex
• Not escaping special characters

4.

Identify the error in this PHP code snippet for validating an integer:

$num = '123a';
if (filter_var($num, FILTER_VALIDATE_INT)) {
    echo 'Valid integer';
} else {
    echo 'Invalid integer';
}

medium

A. filter_var returns false for '123a', so output is 'Invalid integer'

B. filter_var should use FILTER_VALIDATE_NUMBER instead

C. The variable $num should be cast to int before validation

D. filter_var does not validate integers

Solution

1. Step 1: Understand filter_var with FILTER_VALIDATE_INT

   This filter returns the integer value if valid, or false if invalid.

2. Step 2: Check input '123a'

   The string '123a' is not a valid integer, so filter_var returns false.

3. Final Answer:

   filter_var returns false for '123a', so output is 'Invalid integer' -> Option A

4. Quick Check:

   Invalid integer string returns false = 'Invalid integer' output [OK]

Hint: filter_var returns false if input is not a pure integer string [OK]

Common Mistakes:

• Using FILTER_VALIDATE_NUMBER which does not exist
• Casting before validation causing wrong results
• Assuming filter_var always returns boolean

5.

You want to validate a username in PHP that must start with a letter, can contain letters, numbers, underscores, and be 5 to 15 characters long. Which preg_match pattern correctly validates this?

hard

A. /^[a-zA-Z][a-zA-Z0-9_]{4,14}$/

B. /^[0-9][a-zA-Z0-9_]{4,14}$/

C. /^[a-zA-Z0-9_]{5,15}$/

D. /^[a-zA-Z]{5,15}$/

Solution

1. Step 1: Understand username requirements

   Username must start with a letter, then 4 to 14 characters of letters, digits, or underscores, total length 5-15.

2. Step 2: Analyze each pattern

   /^[a-zA-Z][a-zA-Z0-9_]{4,14}$/ matches a letter at start, then 4-14 allowed chars, total 5-15 length. Others fail start or length rules.

3. Final Answer:

   /^[a-zA-Z][a-zA-Z0-9_]{4,14}$/ -> Option A

4. Quick Check:

   Start letter + allowed chars + length 5-15 = /^[a-zA-Z][a-zA-Z0-9_]{4,14}$/ [OK]

Hint: Start with letter, then allowed chars with correct length in regex [OK]

Common Mistakes:

• Allowing digits as first character
• Not enforcing length limits
• Missing anchors ^ and $