Accessing Your MySQL Database from the Web with PHP [con't]
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Querying a Database from the Web
In any script used to access a database from the Web, you follow some basic steps:
- Check and filter data coming from the user.
- Set up a connection to the appropriate database.
- Query the database.
- Retrieve the results.
- Present the results back to the user.
These are the steps we followed in the script results.php, so now let's go through each of them in turn.
Checking and Filtering Input Data
You begin the script by stripping any whitespace that the user might have inadvertently entered at the beginning or end of his search term. You do this by applying the function trim() to the value of $_POST['searchterm'] when giving it a shorter name:
The next step is to verify that the user has entered a search term and selected a search type. Note that you check whether the user entered a search term after trimming whitespace from the ends of $searchterm. If you arrange these lines in the opposite order, you could encounter situations in which a user's search term is not empty and therefore does not create an error message; instead, it is all whitespace, so it is deleted by trim():
You check the $searchtype variable, even though in this case it's coming from an HTML SELECT. You might ask why you should bother checking data that has to be filled in. It's important to remember that there might be more than one interface to your database.For example, Amazon has many affiliates who use its search interface. Also,it's sensible to screen data in case of any security problems that can arise because of users coming from different points of entry.
When you plan to use any data input by a user, you need to filter it appropriately for any control characters. As you might remember, in Chapter 4, "String Manipulation and Regular Expressions," we described the functions addslashes(), stripslashes(), and getmagicquotes_gpc(). You need to escape data when submitting any user input to a database such as MySQL.
In this case, you check the value of the getmagicquotes_gpc() function. It tells you whether quoting is being done automatically. If it is not, you use addslashes() to escape the data:
You also use stripslashes() on the data coming back from the database. If the magic quotes feature is turned on, the data will have slashes in it when it comes back from the database, so you need to take them out.
Here you use the function htmlspecialchars() to encode characters that have special meanings in HTML. The current test data does not include any ampersands (&), less than (<), greater than (>), or double quotation mark (") symbols, but many fine book titles contain an ampersand. By using this function, you can eliminate future errors.
Setting Up a Connection
The PHP library for connecting to MySQL is called mysqli (the i stands for improved). When using the mysqli library in PHP, you can use either an object-oriented or procedural syntax.
You use the following line in the script to connect to the MySQL server:
This line instantiates the mysqli class and creates a connection to host localhost with username bookorama, and password bookorama123. The connection is set up to use the database called books.
Using this object-oriented approach, you can now invoke methods on this object to access the database. If you prefer a procedural approach, mysqli allows for this, too. To connect in a procedural fashion, you use
This function returns a resource rather than an object. This resource represents the connection to the database, and if you are using the procedural approach, you will need to pass this resource in to all the other mysqli functions. This is very similar to the way the file-handling functions, such as fopen(), work.
Most of the mysqli functions have an object-oriented interface and a procedural interface. Generally, the differences are that the procedural version function names start with mysqli and require you to pass in the resource handle you obtained from mysqliconnect(). Database connections are an exception to this rule because they can be made by the mysqli object's constructor.
The result of your attempt at connection is worth checking because none of the rest of code will work without a valid database connection. You do this using the following code:
(This code is the same for the object-oriented and procedural versions. ) The mysqliconnecterrno() function returns an error number on error, or zero on success.
Note that when you connect to the database, you begin the line of code with the error suppression operator, @. This way, you can handle any errors gracefully. (This could also be done with exceptions, which we have not used in this simple example. )
Bear in mind that there is a limit to the number of MySQL connections that can exist at the same time. The MySQL parameter max_connections determines what this limit is. The purpose of this parameter and the related Apache parameter MaxClients is to tell the server to reject new connection requests instead of allowing machine resources to be completely used up at busy times or when software has crashed.
You can alter both of these parameters from their default values by editing the configuration files. To set MaxClients in Apache, edit the httpd.conf file on your system. To set max_connections for MySQL, edit the file my.conf.
