Requirements

[edit] PHP

PHP 4.2.0 or newer (PHP5 information), PHP 5.2.0 with the Standard PHP Library (SPL) extension enabled for the upcoming Pma 3

Session support. Make sure that sessions are configured properly (see below) and session.cookie_secure is Off if you are not using a secure connection (i.e. "https")
Either the mysql or mysqli extensions (more details)

The mbstring extension
GD2 support in PHP to display inline thumbnails of JPEGs ("image/jpeg: inline") with their original aspect ratio

PHP 4.3.0 or newer is needed to use the "text/plain: external" MIME-based transformation

If you are using the Hardened PHP Suhosin extension make sure to increase the values for suhosin.request|get|post.max_* see also Suhosin Configuration and Hardened-PHP request limits. You can examine some increased suhosin.x values on the demo server -> main page -> Show PHP information, e.g., suhosin.post.max_value_length and suhosin.request.max_value_length = 128000 . Another such obstacle is "mod_security" on fedora boxes, error: "Method Not Implemented. GET ..."; a possible solution: http://forums.theplanet.com/index.php?showtopic=89327 .

If any paths are defined in your php.ini like session.save_path or upload_tmp_dir make sure that they do exist and your os is able to write to them. Especially on a Windows system "/tmp" does not exist by default! Due to a broken WIN PHP 5.2 installer paths are definitely wrong in that version bugs.php.net/bug.php?id=39842. Using IIS as webserver you also need an user named "IUSR" with r/w/modify rights granted on those folders.

Not really a requirement but a strong suggestion: if you are using the cookie authentication method, having the mcrypt PHP extension on your web server accelerates not only the login phase but every other action that you do in phpMyAdmin

On 64 bit systems the mcrypt PHP extension is required for auth_type "cookie". The alternative blowfish routines only work on 32-bit systems at the moment (due to a PHP 5.2.0/1 bug, should be fixed in PHP 5.2.2 meanwhile). The same issue leads to the cryptic "#1045 access denied for user www-data.." messages on Ubuntu boxes.