Introduction

Thanks to the PHP-FPM project and major distros picking it up (usually as php5-fpm) newer shared hosting setups provide a set of per-user pools of PHP-FPM processes, combining the advantages of a running PHP process (caching of compiled version of .php files) and SuExec-style execution of PHP scripts under specific low-right system user (protecting shared hosting customers' data from each other).

However, there about 10 different approaches to connect Apache with PHP5-FPM, and about 1000 HowTos ...

SetHandler approach

From Apache 2.4.10 (?) on it's simple. Actually painfully simple compared to earlier approaches:

# i.e. in VirtualHost
<IfModule mod_fastcgi.c>
<FilesMatch \.php$>
    SetHandler "proxy:fcgi://127.0.0.1:9000/"
    # or with socket
    # SetHandler "proxy:unix:/var/run/php5-fpm.sock|fcgi://localhost"
</FilesMatch>
</IfModule>

Based on  http://blog.remirepo.net/post/2014/03/28/PHP-FPM-and-HTTPD-2.4-improvement

You may have to enable the proxy_fcgi module.

Advantages

Simple.

Should be evaluated late enough to allow for heavy .htaccess Rewrite orgies.

More secure? (I think FilesMatch only to existing files that really end in ".php", but I'm not 100% sure yet)

Disadvantages

HTTP Basic Authentication does not work for PHP scripts. Cookie auth works.

ProxyPass approach

To avoid what looks like a "subdirectory move" to Rewrite, one might use ProxyPassMatch (or LocationMatch + ProxyPass) instead of FastCgiExternalServer and Action, as proposed in https://wiki.apache.org/httpd/PHP-FPM

Advantages

Works before Apache 2.4.10

Disadvantages

May be evaluated too early, circumventing .htaccess Rewrite orgies.

Security risks, see bottom of https://wiki.apache.org/httpd/PHP-FPM

Not sure if HTTP Basic Authentication works for PHP script.

FastCgiExternalServer / Action approach

Advantages 

Works before Apache 2.4.10

HTTP Basic Authentication works for PHP scripts.

Definitely evaluated late enough to allow for .htaccess Rewrite orgies, but works as a Rewrite itself which can lead to Rewrite loops.

Disadvantages

Complicated to configure. 

Works as a Rewrite itself which can lead to Rewrite loops.

Details

This was the first almost-satisfying approach I found, but it has major disadvantages.

A typical setup may look like this:

# once for each DocumentRoot
<IfModule mod_fastcgi.c>
  FastCgiExternalServer /home/jdoe/www.jdoe/php5-fcgi -socket /var/run/php5-fpm-jdoe.sock -pass-header Authorization
</IfModule>
 
# once for each VirtualHost, often twice for :80 and :443
<IfModule mod_suexec.c>
  SuexecUserGroup jdoe doe
</IfModule>
<IfModule mod_fastcgi.c>
  AddType application/x-httpd-fastphp5 .php
  Action application/x-httpd-fastphp5 /php5-fcgi
</IfModule>

Unfortunatley, the Rewrite engine can get confused by this, because it sees requests for say /foobar.php scripts with a path like /php5-fcgi/foobar.php.

This can trigger Rewrite "loops", producing a 500 response code for the client (browser) and something like this in the Apache error.log:

AH00124: Request exceeded the limit of 10 internal redirects due to probable configuration error. Use 'LimitInternalRecursion' to increase the limit if necessary. Use 'LogLevel debug' to get a backtrace., referer: https://.../wp-admin/

Further Reading: Action directive, FastCgiExternalServer directive.

Circumventing Rewrite loops, general approach

Exclude /php5-fcgi (or whatever virtual path the LAMP setup uses) from problematic RewriteRules:

# before RewriteRule:
RewriteCond %{REQUEST_URI} !^/php5-fcgi/*

Circumventing Rewrite loops in typical .htaccess WordPress

A typical .htaccess for WordPress looks like this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

To avoid the rewrite loop, change it to this:

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_URI} !^/php5-fcgi/*
RewriteRule . /index.php [L]
</IfModule>
# END WordPress