There are 2 repositories under php-webshells topic.
绕过专业工具检测的Webshell研究文章和免杀的Webshell
Collection of various files from infected hosts
Our powerful php Webshell created by TermuxHackz Team members
基于机器学习的 Webshell 检测 (OPCode -> N-Gram -> TF-IDF -> XGBoost)
Wordpress capable PHP backdoors