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In this example we will learn how to properly connect to Mysql database using PDO. It is based on the information provided in the main article on PDO but with additional explanations.

Surprisingly, there is no single state-of-the-art connection example in the PHP manual. Instead, different connection options are discussed in different chapters, which makes it hard for the learner to get a single robust example that is ready to use. Below you will find such an example, as well as the explanation of all the options used.

Example

Credentials exlained

First of all we are defining variables that contain connection credentials. This set is familiar to anyone who were using the old mysql_connect() function, save for $charset may be, which was rarely used (although it should have been).

  • $host stands for the database host. In case of the local development, it is most likely be 127.0.0.1 or localhost. In case of the live site, the actual hostname should be provided by the site admin / hosting provider. Note that connecting through IP address could save you a headache or two, so if you have a trouble with 'localhost', try to use 127.0.0.1 instead.
  • $db is the name of the database in MySQL (the value that you were passing into mysql_select_db()). On your local server it could be anything, while on a live site again it should be given to you by the admin / provider.
  • $user - a database user
  • $pass - a database password
  • $charset is a very important option. It is telling the database in which encoding you are sending the data in and would like to get the data back. Note that due to initially limited support of unicode in the utf8 MySQL charset, it is now recommended to use utf8mb4 instead.

Note it's a good idea to store connection variables ($host, $db etc.) in a separate file. This way you'll be able to have two versions of your code, one for the local server and one for the remote.

Connection options explained

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Next we are creating an array with PDO options that are either critically important or just make your experience with PDO much better.

  • PDO::ATTR_ERRMODE - this is a cornerstone option that should be always set to PDO::ERRMODE_EXCEPTION. It tells PDO to throw an exception every time a query failed, so you won't have to get the error manually after every query call as it was used to be with mysql_query()
  • PDO::ATTR_EMULATE_PREPARES - this option tell PDO whether to use an emulation mode or not. It is agreed upon that in general it's better to turn it off, however in some cases it is convenient to have it turned on. Luckily, this setting could be changed in runtime using PDO::setAttribute() method, so let's make it turned off by default as a connection option, with the possibility to fall back later.
  • PDO::ATTR_DEFAULT_FETCH_MODE - this option is used simply for convenience. Although the fetch method can be always set right in the fetch function call (like $row = $stmt->fetch(PDO::FETCH_ASSOC);), it is convenient to set it once for all and then just omit it in particular fetches. Besides, when iterating over a statement using foreach there is no place where we can set the fetch mode, so again it is convenient to set it beforehand. The two most popular fetch modes are PDO::FETCH_ASSOC and PDO::FETCH_OBJ which make PDO to fetch the resulting row as an associative array or as an object.

Handling errors

An uncaught exception is converted to a PHP fatal error. It is not a problem by itself, errors are for the good, and we desperately need this one to get the idea what's wrong with our database. But such a converted error contains a stack trace added to the error message, which in case of PDO connection error would include the constructor parameters which happen to be the database credentials. Again, it shouldn't be a problem, as on a live site displaying errors should be always turned off anyway, but we are humans and we make mistakes. So, to avoid even a chance to reveal the credentials, we are catching the Exception and immediately re-throwing them. So, now the stack trace begins on the throw line and doesn't contain the database credentials.

Creating the connection

Having all the options and credentials set, we can finally proceed to creating a connection. To do so we need to create an instance of PDO class for which we need to supply 4 parameters of which the first one, called 'DSN' being most important.

DSN is a semicolon-delimited string, consists of param=value pairs, that begins from the driver name and a colon:

Note that it's important to follow the proper format - no spaces or quotes or other decorations have to be used in DSN, but only parameters, values and delimiters, as shown in the manual.

Beside DSN, we are using $user, $pass and $options variables defined above.

We are wrapping the creation of the PDO instance into a try..catch statement in order to be aware of the possible error, but without the risk of revealing the database credentials.

Don'ts

Beside things that you should do, there are always things that you should do not. Some of them we will discuss below.

  • PDO::ATTR_PERSISTENT. Although this option is rather popular in the copy-pasted cargo cult PDO examples, a learner should always avoid it. There are too many drawbacks (that are outside of the scope of this article) whereas no advantages for a small site at all. This option should be used after a strong consideration only, and by no means as a blindly copy-pasted option just in case.
  • using die(), echo or any other output operator for the caught exception. As it is explained in the relevant article, PHP error reporting, an error message should never be printed unconditionally, but only according to the site-wide settings. And this should be done in the site-wide handler only.

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Accessing the newly created connection

There is one thing that makes PDO more complex than old mysql_connect related stuff. Although one was able to use mysql_query anywhere in the code, without taking care of the connection which was magically supplied by PHP, with PDO one should always make sure that once created PDO instance is available in each part of their script.

There are many different strategies to have this done, but mostly used are:

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  • global. Although this method is unanimously frowned upon, in case your code is the usual procedural spaghetti so familiar to every PHP user, using global to access a PDO instance would be your least problem. So for the simplest method possible just create a PHP file with the code above, and then include in the every PHP script that needs a database connection. Then use $pdo variable everywhere you need. To make it accessible in functions, add global $pdo; inside.
  • singleton approach is better but still frowned upon. But at least it can save you a hassle with global variables. An example can be found in the corresponding chapter about a simple PDO wrapper
  • constructor parameter is the most robust method in case your code is OOP. For the every class that needs a database connection, make PDO a constructor parameter

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