I personally use the default .httaccess supplied by Zend. I've another route defined during my apache configs to ensure that everything posted to /api.php/WHATEVER has been routed to a different framework.

Basically add the default routes in to the .htaccess file, this rule has been overwritten. Things I attempted to complete to disable the rewrite engine was the next:

RewriteEngine On
# api.php
RewriteCond %{REQUEST_URI} !^/(api\.php/.*)$
# Existing file
RewriteCond %{REQUEST_FILENAME} -s [OR]
# Existing directory
RewriteCond %{REQUEST_FILENAME} -l [OR]
# Symbolic link
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^.*$ index.php [NC,L]

Regrettably, I do not begin to see the error. Interesting help.

Maybe this will work :

RewriteEngine On
# api.php
RewriteCond %{REQUEST_URI} ^/(api\.php/.*)$
RewriteRule  api.php?(.*) [R=301,L]
# Existing file
RewriteCond %{REQUEST_FILENAME} -s [OR]
# Existing directory
RewriteCond %{REQUEST_FILENAME} -l [OR]
# Symbolic link
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^.*$ - [NC,L]
RewriteRule ^.*$ index.php [NC,L]

however, you may adapt...