In case, the server has the default version of php  5.6, but Roundcube Webmail 1.6.6 requires something higher php 7.3
 

Then we must install additional php, e.g. php 8.1

yum-config-manager –disable ‘remi-php*’

yum-config-manager –enable remi-safe

 yum -y install php81

yum install php81-php-{cli,fpm,mysqlnd,zip,devel,gd,mbstring,curl,xml,pear,bcmath,json,soap,intl,opcache,pcntl,sockets,sysvsem,sysvshm,mbregex,gd,openssl,Kerberos,zlib,calendar,bz2,webp,jpeg,enchant,pspell,sqlite3,tidy,xsl,common,readline,xml,freetype,exif,ftp,gettext,mhash,imap,imap-ssl,pcntl,mysqli}

php81 --version

php81 --modules

systemctl restart php81-php-fpm

systemctl status php81-php-fpm

PHP Name - PHP 8.1.16

Path to the PHP FastCGI binary: /opt/remi/php81/root/usr/bin/php-cgi

Path to the php.ini directory: /etc/opt/remi/php81

     Path to the PHP-FPM init script: /opt/remi/php81/root/usr/sbin/php-fpm

     Path to the php.ini directory: /etc/opt/remi/php81

     Path to the PHP-FPM pool directory: /etc/opt/remi/php81/php-fpm.d

     PHP-FPM socket directory: <empty>

nano /etc/opt/remi/php81/php-fpm.d/www.conf

listen = 127.0.0.1:8993

php_value[session.save_path]    = /var/opt/remi/php81/lib/php/session

php_value[soap.wsdl_cache_dir]  = /var/opt/remi/php81/lib/php/wsdlcache

;php_value[opcache.file_cache]  = /var/opt/remi/php81/lib/php/opcache

Modify roundcube configuration

/etc/httpd/conf.d/roundcubemail.conf

php-fpm

<Directory /usr/share/roundcubemail/>
        Options none
        AllowOverride Limit
        Require all granted
        
        <FilesMatch \.php$>
         SetHandler "proxy:fcgi://127.0.0.1:8993"
       </FilesMatch>

</Directory>

FastCGI

<Directory /usr/share/roundcubemail/>
 #       Options none
         Options +ExecCGI
        AllowOverride Limit
        Require all granted
       FCGIWrapper /opt/remi/php81/root/usr/bin/php-cgi .php
        
        <FilesMatch \.php$>
         SetHandler fcgid-script
       </FilesMatch>

</Directory>