How do you enable mod_rewrite on any OS?

If I understand correctly, I need to put something in httpd.config to enable mod_rewrite. If this is true, what do I need to put in httpd.conf or apache.conf? Please be OS specific.

Nope, mod_rewrite is an Apache module and has nothing to do with PHP.

To activate the module, the following line in httpd.conf needs to be active:

LoadModule rewrite_module modules/mod_rewrite.so

to see whether it is already active, try putting a .htaccess file into a web directory containing the line

RewriteEngine on

if this works without throwing a 500 internal server error, and the .htaccess file gets parsed, URL rewriting works.

Just a fyi for people enabling mod_rewrite on Debian with Apache2:

To check whether mod_rewrite is enabled:

Look in mods_enabled for a link to the module by running

ls /etc/apache2/mods-enabled | grep rewrite

If this outputs rewrite.load then the module is enabled. (Note: your path to apache2 may not be /etc/, though it's likely to be.)

To enable mod_rewrite if it's not already:

Enable the module (essentially creates the link we were looking for above):

a2enmod rewrite

Reload all apache config files:

service apache2 restart

In my case, issue was occured even after all these configurations have done (@Pekka has mentioned changes in httpd.conf & .htaccess files). It was resolved only after I add

<Directory "project/path">
  Order allow,deny
  Allow from all
  AllowOverride All
</Directory>

to virtual host configuration in vhost file

Edit on 29/09/2017 (For Apache 2.4 <) Refer this answer

<VirtualHost dropbox.local:80>
DocumentRoot "E:/Documenten/Dropbox/Dropbox/dummy-htdocs"
ServerName dropbox.local
ErrorLog "logs/dropbox.local-error.log"
CustomLog "logs/dropbox.local-access.log" combined
<Directory "E:/Documenten/Dropbox/Dropbox/dummy-htdocs">
    # AllowOverride All      # Deprecated
    # Order Allow,Deny       # Deprecated
    # Allow from all         # Deprecated

    # --New way of doing it
    Require all granted    
</Directory>