You will need to install the DataStax C/C++ driver which is a dependency of the PHP driver. Using these instructions followed by make install after the driver has been successfully built will ensure this dependency is available when building the PHP driver. Using the PHP driver build instructions you will need to make sure that GMP and PHP dev libraries are also available before running pecl install cassandra.

EDIT:

Since you are using El Capitan you are running into issues with the System Integrity Protection and you will need to disable it in order to copy files into /usr. The better and recommended option is to install PHP using Homebrew; however you can also use MacPorts if preferred.

Below are the steps used to reproduce the installation of the PHP driver on a clean OSX El Capitan image with Xcode and Homebrew already installed:

brew install autoconf cmake libuv gmp openssl pcre homebrew/php/php55
brew link homebrew/php/php55
mkdir code
pushd code
git clone https://github.com/datastax/php-driver.git
pushd php-driver
git submodule update --init --recursive
pushd lib/cpp-driver
mkdir build
pushd build
cmake -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl ..
make -j$(sysctl -n hw.ncpu)
sudo make install
popd
popd
mkdir build
pushd ext
/usr/local/bin/phpize
popd
pushd build
../ext/configure --with-php-config=/usr/local/bin/php-config
make -j$(sysctl -n hw.ncpu)
sudo make install
popd
popd
sudo sh -c 'echo "extension=cassandra.so" >> /usr/local/etc/php/5.5/php.ini'

You can then verify the installation using the following command:

/usr/local/bin/php -i | grep -A 10 "^cassandra$"

NOTE: PHP v5.5 is utilized above since that is the default version that comes with El Capitan; PHP v5.6 and v7.0 can also be used instead.

In command prompt type below

set PATH=%PATH%;C:pathtophp

Where C:pathtophp is the folder where your php.exe file is located. After this run

C:>php -v

than you will see something like

PHP 7.3.6 (cli) (built: May 29 2019 12:11:00) 

UPDATE:

If you find that when you restart your computer, this solution will not work anymore:

Set the Path like as following:

Step 1 - Click on the Windows icon

Step 2 - Click on the Settings icon

Step 3 - Click on System

Step 4 - Click on About

Step 5 - Click on System info

Step 6 - Click on Advanced system settings

Step 7 - Click on Environment variables...

Step 8 - Select Path row and then click Edit

Step 9 - Click New and then click Browse, then in the next panel which will open you need to select the folder you want in the Path. For the initial premise of this guide i will add the folder C:UsersNewFolderInPath♥

Note: Replace your PHP installed path(e.g:C:Program Filesphp) with the above path ♥.

Step 10 - Click OK and click every OK button you will encounter to close every previous windows.