Merge pull request #1236 from infosiftr/php-m

Add a short note about checking "php -m" before compiling more extensions

php/content.md

@@ -89,6 +89,8 @@ RUN sed -ri -e 's!/var/www/!${APACHE_DOCUMENT_ROOT}!g' /etc/apache2/apache2.conf
 
 ## How to install more PHP extensions
 
+Many extensions are already compiled into the image, so it's worth checking the output of `php -m` or `php -i` before going through the effort of compiling more.
+
 We provide the helper scripts `docker-php-ext-configure`, `docker-php-ext-install`, and `docker-php-ext-enable` to more easily install PHP extensions.
 
 In order to keep the images smaller, PHP's source is kept in a compressed tar file. To facilitate linking of PHP's source with any extension, we also provide the helper script `docker-php-source` to easily extract the tar or delete the extracted source. Note: if you do use `docker-php-source` to extract the source, be sure to delete it in the same layer of the docker image.