Thanks for your help.
I just installed php-cgi and uninstalled libapache2-mod-php. But still no difference.
I uninstalled every php package that is not marked as dependency for checkmk, there is not much left now:
dpkg -l | grep php | grep ii
ii libapache2-mod-php7.4 7.4.3-4ubuntu2.4 amd64 server-side, HTML-embedded scripting language (Apache 2 module)
ii php-common 2:75 all Common files for PHP packages
ii php-pear 1:1.10.9+submodules+notgz-1ubuntu0.20.04.2 all PEAR Base System
ii php7.4-cgi 7.4.3-4ubuntu2.4 amd64 server-side, HTML-embedded scripting language (CGI binary)
ii php7.4-cli 7.4.3-4ubuntu2.4 amd64 command-line interpreter for the PHP scripting language
ii php7.4-common 7.4.3-4ubuntu2.4 amd64 documentation, examples and common module for PHP
ii php7.4-dev 7.4.3-4ubuntu2.4 amd64 Files for PHP7.4 module development
ii php7.4-gd 7.4.3-4ubuntu2.4 amd64 GD module for PHP
ii php7.4-json 7.4.3-4ubuntu2.4 amd64 JSON module for PHP
ii php7.4-ldap 7.4.3-4ubuntu2.4 amd64 LDAP module for PHP
ii php7.4-opcache 7.4.3-4ubuntu2.4 amd64 Zend OpCache module for PHP
ii php7.4-readline 7.4.3-4ubuntu2.4 amd64 readline module for PHP
ii php7.4-soap 7.4.3-4ubuntu2.4 amd64 SOAP module for PHP
ii php7.4-sqlite3 7.4.3-4ubuntu2.4 amd64 SQLite3 module for PHP
ii php7.4-xml 7.4.3-4ubuntu2.4 amd64 DOM, SimpleXML, XML, and XSL module for PHP
ii php7.4-xmlrpc 7.4.3-4ubuntu2.4 amd64 XMLRPC-EPI module for PHP
ii pkg-php-tools 1.38 all various packaging tools and scripts for PHP packages