5 Ways to Implement Email Exists Validation in PHP

Preventing duplicate accounts by checking if an email already exists is a fundamental part of user registration. We'll explore five ways to handle this in PHP, complete with code snippets. We will also cover the pitfalls of traditional methods and show how Abstract API addresses these issues.

How to Implement Email Exists Validation in PHP

Here are four common methods to check for duplicate emails in your PHP application. Each approach uses a different strategy to query the database or an in-memory store.

Pre-flight SELECT with a Prepared Statement

This method involves a pre-flight query to check if an email exists before an INSERT operation. The code prepares a statement that looks for a match in the users table. It uses fetchColumn() to get a single column, and if a record is found, the function returns a value that indicates a duplicate.

$stmt = $pdo->prepare('SELECT 1 FROM users WHERE email = ? LIMIT 1');
$stmt->execute([$email]);
if ($stmt->fetchColumn()) { 
    // duplicate 
}

Unique Constraint and Exception Handling

This approach relies on the database to enforce uniqueness. First, you apply a UNIQUE constraint to the email column. The application then attempts to insert a new user record without a prior check. If the email exists, the database rejects the INSERT and generates a duplicate-key error.

The code, inside a try...catch block, catches this PDOException. It then inspects the error information to confirm it is a duplicate entry error, like MySQL's error code 1062, a common topic on PHP forums and in other developer communities.

try {
    $stmt = $pdo->prepare('INSERT INTO users(email, pwd) VALUES(?, ?)');
    $stmt->execute([$email, $hash]);
} catch (PDOException $ex) {
    if ($ex->errorInfo[1] == 1062) { 
        // duplicate email 
    }
    else { throw $ex; }
}

Atomic UPSERT Operation

An UPSERT operation combines an INSERT and an UPDATE into a single, atomic command. The database attempts to insert a new row. If this action violates a unique constraint, it performs an UPDATE instead.

The specific syntax varies between database systems. For MySQL, the INSERT ... ON DUPLICATE KEY UPDATE statement is used. After execution, you check the number of affected rows. A rowCount() of 2 in MySQL signals a duplicate.

$stmt = $pdo->prepare('INSERT INTO users(email,pwd) VALUES(?,?) ON DUPLICATE KEY UPDATE id=id');
$stmt->execute([$email, $hash]);
$duplicate = ($stmt->rowCount() == 2);

In-Memory Reservation with Redis

This technique uses an in-memory data store like Redis to manage email reservations. Before any database interaction, the application calls the SETNX command in Redis.

This command attempts to set a key, for example "email:user@example.com", and only succeeds if the key does not already exist. If SETNX returns 0, the email is already reserved.

If it succeeds, the application can proceed with the database write. The key should have a short Time-To-Live (TTL). After a successful commit, the application should delete the key.

Challenges of Traditional Email Validation

These traditional methods introduce significant drawbacks that can compromise data integrity and user experience. The approaches often overlook critical nuances in how email addresses function in the real world.

• A pre-flight SELECT query creates a race condition. Under load, two requests can see an email as available and then attempt to insert it simultaneously. This makes application-side checks inherently brittle without a database-level unique constraint.
• Email standards and mail systems handle case sensitivity differently. A simple string comparison or forced lower-case conversion can miss duplicates or create false collisions. This affects database query methods which use simple string comparison, like pre-flight SELECT.
• Simple string comparisons fail to account for provider-specific alias rules. For example, Gmail ignores dots and plus-labels in addresses. This oversight, common in all database-centric methods, allows users to create multiple accounts that point to the same actual mailbox.
• Live SMTP checks to verify an address are unreliable. Many mail servers disable verification commands or use catch-all addresses. These probes also vary in response, face interference from greylisting, and can damage your sender reputation.