1.) The easy was is to use composer. It will install the best version for your system, and if you really still using PHP 5.3 then it will work. Take a look at the test results: https://travis-ci.org/voku/anti-xss/builds/371703922
composer require voku/anti-xsscomposer require voku/anti-xss
2.) Or you can install by hand. Not recommended, because you need to install the dependencies also by hand.
github.com/voku/anti-xss/tree/php_o
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