Another point to consider and one of the advantages of PDO is that it's not limited to mysql and supports other database engines such as PostgreSQL, SQLite, etc.
PHP offers three different APIs to connect to MySQL. Below we show the APIs provided by the mysql, mysqli, and PDO extensions. Each code snippet creates a connection to a MySQL server running on "example.com" using the username "user" and the password "password". And a query is run to greet the user.
Example #1 Comparing the three MySQL APIs
<?php
// mysqli
$mysqli = new mysqli("example.com", "user", "password", "database");
$result = $mysqli->query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = $result->fetch_assoc();
echo htmlentities($row['_message']);
// PDO
$pdo = new PDO('mysql:host=example.com;dbname=database', 'user', 'password');
$statement = $pdo->query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = $statement->fetch(PDO::FETCH_ASSOC);
echo htmlentities($row['_message']);
// mysql
$c = mysql_connect("example.com", "user", "password");
mysql_select_db("database");
$result = mysql_query("SELECT 'Hello, dear MySQL user!' AS _message FROM DUAL");
$row = mysql_fetch_assoc($result);
echo htmlentities($row['_message']);
?>
Recommended API
It is recommended to use either the mysqli or PDO_MySQL extensions. It is not recommended to use the old mysql extension for new development, as it was deprecated in PHP 5.5.0 and was removed in PHP 7. A detailed feature comparison matrix is provided below. The overall performance of all three extensions is considered to be about the same. Although the performance of the extension contributes only a fraction of the total run time of a PHP web request. Often, the impact is as low as 0.1%.
Feature comparison
| ext/mysqli | PDO_MySQL | ext/mysql | |
|---|---|---|---|
| PHP version introduced | 5.0 | 5.1 | 2.0 |
| Included with PHP 5.x | Yes | Yes | Yes |
| Included with PHP 7.x | Yes | Yes | No |
| Development status | Active | Active | Maintenance only in 5.x; removed in 7.x |
| Lifecycle | Active | Active | Deprecated in 5.x; removed in 7.x |
| Recommended for new projects | Yes | Yes | No |
| OOP Interface | Yes | Yes | No |
| Procedural Interface | Yes | No | Yes |
| API supports non-blocking, asynchronous queries with mysqlnd | Yes | No | No |
| Persistent Connections | Yes | Yes | Yes |
| API supports Charsets | Yes | Yes | Yes |
| API supports server-side Prepared Statements | Yes | Yes | No |
| API supports client-side Prepared Statements | No | Yes | No |
| API supports Stored Procedures | Yes | Yes | No |
| API supports Multiple Statements | Yes | Most | No |
| API supports Transactions | Yes | Yes | No |
| Transactions can be controlled with SQL | Yes | Yes | Yes |
| Supports all MySQL 5.1+ functionality | Yes | Most | No |
Another point to consider and one of the advantages of PDO is that it's not limited to mysql and supports other database engines such as PostgreSQL, SQLite, etc.
These are quite possibly the most paradoxical lines in this table:
API supports non-blocking, asynchronous queries with mysqlnd
ext/mysqli: Yes PDO_MySQL: No
API supports client-side Prepared Statements
ext/mysqli: No PDO_MySQL: Yes
Apparently it's either asynchronous I/O or the security of bound parameters.
Another useful consideration to keep in mind when choosing your library is how extensible it is. Chances are, in any sufficiently advanced development scenario, you're going to be extending your database access class to add a method (or multiple methods) for how to handle database errors and alert the development team of errors and whether to have the code fail immediately or fail gracefully serving the user a user-friendly failure notice.
For example, I have a class where I have added extra parameters to the query() function (and a few others), which accept the __FILE__ and __LINE__ constants to facilitate tracking issues. If this were not reasonably possible with PDO-mysql for example (not sure, never used it), it may make one option or the other much less likely to be viable for your usage case.
Apart from the feature list, I suggest you try out both MySQLi and PDO and find out what API design you like most. MySQLi is more powerful and probably more complex to learn. PDO is more elegant and has the advantage that you only need to learn one PHP API if you need to work with different DBMS in the future.