Zen Cart v1.2.x and v1.3.x

Minimum/suggested server requirements:

• PHP 4.3.2 or higher (PHP 4.4.x for optimal performance),
  
• Apache 1.3.x and MySQL 3.2.x or higher.
  
• Register Globals may be on or off, with "off" being the recommended configuration
  
• safe_mode off recommended.

NOTE: Many shipping and payment modules require CURL to be installed/compiled with PHP and OpenSSL.

NOTE: Support for https may be required depending on the payment methods being accepted. The use of SSL during account creation and checkout is recommended.

PHP Version

Zen Cart v1.2.x through v1.3.6 are compatible with PHP 4.3.2 - 4.4.x.  They are NOT compatible with PHP 5.
Zen Cart v1.3.7-v1.3.8a are compatible with PHP 5.1.x and PHP 5.2.x.  (And can be adapted to work with PHP 5.3 by applying this patch: http://www.zen-cart.com/forum/showthread.php?t=140960 )
Zen Cart v1.3.9 (to be released soon) and newer are compatible with PHP 5.3.0 and newer.

MySQL Version

MySQL 5 is not officially supported by Zen cart v1.3.x and lower.
However, shopowners running v1.3.6 and newer seem to be reasonably successful.  MANY addons for v1.2.x and v1.3.x are not properly written to MySQL 5 standards, and as such may require alterations if you choose to use them.
Using the latest version of Zen Cart is always recommended for maximum compatibility.

Zen Cart v2.0 has been written with MySQL 5.1 compatibility in mind.  Addon-compatibility may vary.

Zen Cart v2.x

Zen Cart v2.0 (currently in development) will have a different set of minimum requirements:

• PHP 5.2.3 minimum, and compatible with PHP 5.3 and PHP 6.0
  
• CURL-with-SSL support compiled into PHP will be required
• MySQL 4.1 minimum, although MySQL 5 is becoming far more common and expected
• Register-Globals may be on or off, but SHOULD be OFF
  
• PHP safe mode should be OFF
• SSL is recommended