If you're having problems compiling on Linux like this:
checking whether build with IMAP works... no
configure: error: build test failed. Please check the config.log for details.
... then try adding: "--with-openssl" to your switches.
Not quite sure why but here's where I found the idea:
To get these functions to work, you have to compile PHP with --with-imap[=DIR] , where DIR is the c-client install prefix. From our example above, you would use --with-imap=/usr/local/imap-2000b . This location depends on where you created this directory according to the description above. Windows users may include the php_imap.dll DLL in php.ini.
IMAP is not supported on systems earlier than Windows 2000. This is because it uses encryption functions in order to enable SSL connections to the mail servers.
Note: Depending on how the c-client was configured, you might also need to add --with-imap-ssl=/path/to/openssl/ and/or --with-kerberos=/path/to/kerberos to the PHP configure line.