How do I tell Apache which PHP to use?

I think all these answers aren't really answering the question. The root level can be determined by running the command httpd -V. This will show you what options the Apache daemon was built with at compile time. This is what controls where httpd determines where to look for it's config. files and .so modules by default.

For example:

% httpd -V
Server version: Apache/2.2.17 (Unix)
Server built:   Dec 17 2010 11:58:24
Server's Module Magic Number: 20051115:25
Server loaded:  APR 1.3.12, APR-Util 1.3.9
Compiled using: APR 1.3.12, APR-Util 1.3.9
Architecture:   32-bit
Server MPM:     Prefork
  threaded:     no
    forked:     yes (variable process count)
Server compiled with....
 -D APACHE_MPM_DIR="server/mpm/prefork"
 -D APR_HAS_SENDFILE
 -D APR_HAS_MMAP
 -D APR_HAVE_IPV6 (IPv4-mapped addresses enabled)
 -D APR_USE_SYSVSEM_SERIALIZE
 -D APR_USE_PTHREAD_SERIALIZE
 -D SINGLE_LISTEN_UNSERIALIZED_ACCEPT
 -D APR_HAS_OTHER_CHILD
 -D AP_HAVE_RELIABLE_PIPED_LOGS
 -D DYNAMIC_MODULE_LIMIT=128
 -D HTTPD_ROOT="/etc/httpd"
 -D SUEXEC_BIN="/usr/sbin/suexec"
 -D DEFAULT_PIDLOG="logs/httpd.pid"
 -D DEFAULT_SCOREBOARD="logs/apache_runtime_status"
 -D DEFAULT_LOCKFILE="logs/accept.lock"
 -D DEFAULT_ERRORLOG="logs/error_log"
 -D AP_TYPES_CONFIG_FILE="conf/mime.types"
 -D SERVER_CONFIG_FILE="conf/httpd.conf"

The key line in that output is the HTTPD_ROOT. That defines where Apache's ROOT directory is to start, /etc/httpd in my case, when looking for config. files and modules.

NOTE: This ROOT is not the same thing as DocumentRoot. This ROOT is specific to how the httpd daemon was compiled, the DocumentRoot is for specifying where the httpd daemon should start looking for actual web content (.html files and such).

For my httpd.conf file I have the following Load lines:

LoadModule auth_basic_module modules/mod_auth_basic.so
LoadModule auth_digest_module modules/mod_auth_digest.so
LoadModule authn_file_module modules/mod_authn_file.so

Given this the full path to your modules would be, for example:

/etc/httpd/modules/mod_auth_basic.so

This is from a CentOS 5.x system but the technique is still apt.

BTW, it can get a little confusing because in CentOS' case the files are organized physically here:

% ls /usr/lib/httpd/modules/
libphp5.so            mod_authnz_ldap.so      mod_dav_fs.so      mod_headers.so       mod_perl.so            mod_speling.so

...and then accessible to the Apache daemon, httpd, through this path:

% ls -l /etc/httpd/
total 12
drwxr-xr-x 2 root root 4096 Apr 26  2011 conf
drwxr-xr-x 3 root root 4096 Apr 26  2011 conf.d
-rw-r--r-- 1 root root   18 Feb 24  2009 htpasswd
lrwxrwxrwx 1 root root   19 Apr 26  2011 logs -> ../../var/log/httpd
lrwxrwxrwx 1 root root   27 Apr 26  2011 modules -> ../../usr/lib/httpd/modules
lrwxrwxrwx 1 root root   13 Apr 26  2011 run -> ../../var/run

The modules link connects /etc/httpd --> /usr/lib/httpd/modules.

You can find files on your system with the locate command:

# locate libphp5.so

It will print the full paths of all files with that name. I have one at /usr/libexec/apache2/libphp5.so.

The parent directory of modules loaded in httpd.conf (such as: libexec/apache2/libphp5.so) is defined by the ServerRoot directive which by default is typically set to /usr. I wouldn't recommend changing this but it may be useful for someone to know just where exactly that path is defined.

Apache's website says the following about ServerRoot:

Relative paths in other configuration directives (such as Include or LoadModule, for example) are taken as relative to this directory.

additionally the default httpd.conf file comments read:

ServerRoot: The top of the directory tree under which the server's configuration, error, and log files are kept.

Apache should be looking for modules in "/usr/libexec/httpd/". In there you'll find either a file or symlink called "libphp5.so". If it's a symlink, you'll need to relink to the new 5.2.8 libphp5.so, otherwise just copy the 5.2.8 libphp5.so to "/usr/libexec/httpd/" and restart apache with "sudo apachectl restart".

I had a Apache and PHP installed on one of the server. This was installed by the previous sys admin. Both the Apache and PHP was complied from the source. In addition to this there was a default PHP installed. So to know which PHP is used by the Apache. I run the below command

   <Install Dir of PHP>/bin/php -i | grep apxs

This gave me the path to apache apxs

  APACHE_HOME/bin/apxs

This gave me info on which Apache is being used by this php. The default php gave error when i typed

#php -i | grep apxs 

 Failed loading opcache.so:  opcache.so: cannot open shared object file: No    such file or directory PHP Warning:  PHP Startup: Unable to load dynamic library     '<PHP_HOME>/lib/php/extensions/debug-non-zts-20121212/memcached.so' - <PHP_HOME>/lib/php/extensions/debug-non-zts-20121212/memcached.so: undefined symbol: OnUpdateLongGEZero in Unknown on line 0

So in this way i was able to figure out the php used by Apache.