Let's assume that you have installed PHP to /opt/php5/ and ICU is installed to /opt/icu/. Run the following commands:
$ /opt/php5/bin/phpize $ ./configure --with-php-config=/opt/php5/bin/php-config --with-icu-dir=/opt/icu $ make
If your ICU is installed to a non-standard directory then you might want to specify its location in LD_LIBRARY_PATH environment variable so that dynamic linker can find it:
$ export LD_LIBRARY_PATH=/opt/icu/lib
Otherwise, if PHP and ICU are installed to their default locations, then the additional options to `configure' are not needed.