mPDF requires a minimum of PHP 4.3.10 or PHP 5.0.3
mPDF v5.7.1 minimum is required for PHP>=5.5
(Since version 1, mPDF has been developed on PHP5 only, but should still be compatible with PHP4>=4.3.10)
PHP requires the mb_string module to be enabled (including mbregex, which needs to be explicitly enabled in some environments)
mPDF is not compatible with PHP function overloading (mbstring.func_overload)
As for FPDF, PHP requires zlib for compression.
Improved performance requires write permissions to the temporary cache folders.