Writing PHP Scripts | High Performance MySQL: Optimization, Backups, Replication, and More

PHP scripts are plain text files that can contain a mixture of HTML and PHP code. The script is interpreted to produce a Web page as output that is sent to the client. The HTML is copied to the output without interpretation. PHP code is interpreted and replaced by whatever output the code produces.

PHP begins interpreting a file in HTML mode. You can switch into and out of PHP code mode using special tags that signify the beginning and end of PHP code. You can switch between modes any number of times within a file. PHP understands four types of tags, although some of them must be explicitly enabled if you want to use them. One way to do this is by turning them on in the PHP initialization file, php.ini (php3.ini in PHP 3). The location of this file is system dependent; on many UNIX systems, it's found in /usr/lib or /usr/local/lib. On Windows, look in the Windows system directory.

PHP understands the following tag styles:

The default style uses <?php and ?> tags:
```
 <?php print ("Hello, world."); ?>  
```
Short-open-tag style uses <? and ?> tags:
```
 <? print ("Hello, world."); ?>  
```
In PHP 4, this style also allows <?= and ?> tags as a shortcut for displaying the result of an expression without using a print statement:
```
 <?= "Hello, world." ?>  
```
Short tags can be enabled with a directive in the PHP initialization file:
```
 short_open_tag = On;  
```
Active Server Page-compatible style uses <% and %> tags:
```
 <% print ("Hello, world."); %>  
```
In PHP 4, this style also allows <%= and %> tags as a shortcut for displaying the result of an expression without using a print statement:
```
 <%= "Hello, world." %>  
```
ASP-style tags can be enabled with a directive in the PHP initialization file:
```
 asp_tags = On;  
```
Support for ASP tags was introduced in PHP 3.0.4.
If you use an HTML editor that doesn't understand the other tags, you can use <script> and </script> tags:
```
 <script language="php"> print ("Hello, world."); </script>  
```

The examples in this appendix use <?php and ?> for the opening and closing tags.