How can I make WordPress work with a Symfony app?

Update your .htaccess in WordPress root folder with below code

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /foo
RewriteRule ^/foo/app\.php$ - [L]
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress