doc/php: Fix headline conflicts

doc/languages-frameworks/php.section.md

@@ -1,10 +1,8 @@
-# PHP
+# PHP {#sec-php}
 
-## User Guide
+## User Guide {#ssec-php-user-guide}
 
-### Using PHP
-
-#### Overview
+### Overview {#ssec-php-user-guide-overview}
 
 Several versions of PHP are available on Nix, each of which having a
 wide variety of extensions and libraries available.
@@ -36,7 +34,7 @@ opcache extension shipped with PHP is available at
 `php.extensions.opcache` and the third-party ImageMagick extension at
 `php.extensions.imagick`.
 
-#### Installing PHP with extensions
+### Installing PHP with extensions {#ssec-php-user-guide-installing-with-extensions}
 
 A PHP package with specific extensions enabled can be built using
 `php.withExtensions`. This is a function which accepts an anonymous
@@ -89,7 +87,7 @@ php.buildEnv {
 }
 ```
 
-##### Example setup for `phpfpm`
+#### Example setup for `phpfpm` {#ssec-php-user-guide-installing-with-extensions-phpfpm}
 
 You can use the previous examples in a `phpfpm` pool called `foo` as
 follows:
@@ -113,7 +111,7 @@ in {
 };
 ```
 
-##### Example usage with `nix-shell`
+#### Example usage with `nix-shell` {#ssec-php-user-guide-installing-with-extensions-nix-shell}
 
 This brings up a temporary environment that contains a PHP interpreter
 with the extensions `imagick` and `opcache` enabled: