Host two web application on the same domain using Apache

You can use the Alias directive to map all requests matching the pattern /custom-slug/* to the Symfony app. All other URLs will be served from wordpress.

Sample vhost:

<VirtualHost *:80>
        ServerName www.example.com

        ServerAdmin webmaster@localhost
        DocumentRoot /your_web_root/www_example_com/wordpress/

        # All URLs begining with /custom-slug will be fetched from the symfony app
        Alias "/custom-slug" "/your_web_root/www_example_com/symfonyApp/web/"

        <Directory /your_web_root/www_example_com/wordpress/>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                allow from all
        </Directory>

        <Directory /your_web_root/www_example_com/symfonyApp/web/>
                AllowOverride None
                Order Allow,Deny
                Allow from All

                <IfModule mod_rewrite.c>
                    Options -MultiViews
                    RewriteEngine On
                    RewriteCond %{REQUEST_FILENAME} !-f
                    RewriteRule ^(.*)$ app.php [QSA,L]
                </IfModule>
           </Directory>

</VirtualHost>