Try this in your .htaccess file (inside the web directory):
<IfModule mod_rewrite.c>
Options +FollowSymlinks
RewriteEngine On
# Explicitly disable rewriting for front controllers
RewriteRule ^app_dev.php - [L]
RewriteRule ^app.php - [L]
RewriteCond %{REQUEST_FILENAME} !-f
# Change below before deploying to production
#RewriteRule ^(.*)$ /app.php [QSA,L]
RewriteRule ^(.*)$ /app_dev.php [QSA,L]
</IfModule>