Migration from PHP 3 to PHP 4 is relatively easy, and should not require you to change your code in any way. There are minor incompatibilities between the two versions; You may want to check the incompatibilities list to make sure that you're indeed not affected by them (the chances you're affected by these incompatibilities are extremely slim).
Recent operating systems provide the ability to perform versioning and scoping. This features make it possible to let PHP 3 and PHP 4 run as concurrent modules in one Apache server.
This feature is known to work on the following platforms:
Linux with recent binutils (binutils 18.104.22.168.25 tested)
Solaris 2.5 or better
FreeBSD (3.2, 4.0 tested)
To enable it, configure PHP3 and PHP4 to use APXS (--with-apxs) and the necessary link extensions (--enable-versioning). Otherwise, all standard installations instructions apply. For example:
The global configuration file, php3.ini, has changed its name to php.ini.
The MIME types recognized by the PHP module have changed.
application/x-httpd-php3 --> application/x-httpd-php application/x-httpd-php3-source --> application/x-httpd-php-source
You can make your configuration files work with both versions of PHP (depending on which one is currently compiled into the server), using the following syntax:
AddType application/x-httpd-php3 .php3 AddType application/x-httpd-php3-source .php3s AddType application/x-httpd-php .php AddType application/x-httpd-php-source .phps
In addition, the PHP directive names for Apache have changed.
Starting with PHP 4.0, there are only four Apache directives that relate to PHP:
php_value [PHP directive name] [value] php_flag [PHP directive name] [On|Off] php_admin_value [PHP directive name] [value] php_admin_flag [PHP directive name] [On|Off]
There are two differences between the Admin values and the non admin values:
Admin values (or flags) can only appear in the server-wide apache configuration files (e.g., httpd.conf).
Standard values (or flags) cannot control certain PHP directives, for example - safe mode (if you could override safe mode settings in .htaccess files, it would defeat safe-mode's purpose). In contrast, Admin values can modify the value of any PHP directive.
To make the transition process easier, PHP 4.0 is bundled with scripts that automatically convert your Apache configuration and .htaccess files to work with both PHP 3.0 and PHP 4.0. These scripts do NOT convert the mime type lines! You have to convert these yourself.
To convert your Apache configuration files, run the apconf-conv.sh script (available in the scripts/apache/ directory). For example:
Your original configuration file will be saved in httpd.conf.orig.
To convert your .htaccess files, run the aphtaccess-conv.sh script (available in the scripts/apache/ directory as well):
Likewise, your old .htaccess files will be saved with an .orig prefix.
The conversion scripts require awk to be installed.