mPDF

mPDF was originally developed and tested on PHP 5.1.4 (Windows), and PHP 4.4.4 (Linux),and is currently being developed on PHP 5.2.9 (Windows), and PHP 5.2.10 and 5.2.12 (Linux).

Output files have been tested in Adobe Reader 5, 7, 8 and 9.

Since version 1.1+ mPDF has been developed on PHP5 only, although I am not aware of anything that would be incompatible with PHP4.

PHP requires the mb_string module to be enabled (see http://uk.php.net/manual/en/ref.mbstring.php) which means that PHP>=4.3.0 will be required as a minimum, and PHP 4.3.0 to 4.3.3 need --enable-mbstring=all

The /u modifier in PCRE is required. This modifier is available from PHP 4.1.0 or greater on Unix and from PHP 4.2.3 on win32. UTF-8 validity of the pattern is checked since PHP 4.3.5. and will result in death of the script if invalid utf-8 characters/string is sent to mPDF.

As for FPDF, PHP requires zlib for compression.