Common validation patterns in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is input validation in PHP?

Input validation is the process of checking user input to ensure it meets expected formats and rules before processing it. This helps prevent errors and security issues.

beginner

How do you validate an email address in PHP?

Use the filter_var() function with FILTER_VALIDATE_EMAIL to check if a string is a valid email format.

beginner

What PHP function helps validate if a string contains only letters and numbers?

ctype_alnum() checks if all characters in the string are letters or digits.

intermediate

Why is it important to validate data on the server side in PHP?

Server-side validation ensures data is safe and correct even if client-side validation is bypassed. It protects against malicious input and errors.

beginner

What is a common pattern to validate a required field in PHP?

Check if the field is set and not empty using isset() and !empty() functions before processing.

Which PHP function validates an email format?

A is_numeric()

B ctype_alpha()

C filter_var() with FILTER_VALIDATE_EMAIL

D preg_match() with /[0-9]/

filter_var() with FILTER_VALIDATE_EMAIL checks if the input is a valid email address.

How do you check if a string contains only letters and numbers in PHP?

A filter_var()

B is_string()

C strlen()

D ctype_alnum()

ctype_alnum() returns true if all characters are letters or digits.

What does the PHP function isset() check?

A If a variable is set and not NULL

B If a string is empty

C If a number is positive

D If an array contains values

isset() returns true if the variable exists and is not NULL.

Why should validation be done on the server side?

A To prevent bypassing client-side checks and protect data

B To make the page load faster

C To improve CSS styling

D To reduce server load

Server-side validation ensures security and data integrity even if client-side validation is skipped.

Which PHP function can be used to check if a string matches a pattern?

A strlen()

B preg_match()

C is_array()

D trim()

preg_match() checks if a string matches a regular expression pattern.

Explain how to validate an email address in PHP and why it is important.

Think about built-in PHP functions for filtering.

Describe the difference between client-side and server-side validation and why server-side validation is necessary.

Consider what happens if a user disables JavaScript.

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 $