Extract composer install to separate script

Author: alcaeus

.evergreen/install-dependencies.sh

@@ -38,6 +38,26 @@ install_extension ()
    fi
 
    sudo cp ${PROJECT_DIRECTORY}/.evergreen/config/php.ini ${PHP_PATH}/lib/php.ini
+
+   php --ri mongodb
+}
+
+install_composer ()
+{
+  EXPECTED_CHECKSUM="$(php -r 'copy("https://composer.github.io/installer.sig", "php://stdout");')"
+  php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
+  ACTUAL_CHECKSUM="$(php -r "echo hash_file('sha384', 'composer-setup.php');")"
+
+  if [ "$EXPECTED_CHECKSUM" != "$ACTUAL_CHECKSUM" ]; then
+    >&2 echo 'ERROR: Invalid installer checksum'
+    rm composer-setup.php
+    exit 1
+  fi
+
+  php composer-setup.php --quiet
+  RESULT=$?
+  rm composer-setup.php
+  exit $RESULT
 }
 
 # Functions to fetch MongoDB binaries
@@ -88,12 +108,6 @@ OLD_PATH=$PATH
 PATH=$PHP_PATH/bin:$OLD_PATH
 
 install_extension
-
-php --ri mongodb
-
-# Install composer
-php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
-php composer-setup.php
-php -r "unlink('composer-setup.php');"
+install_composer
 
 php composer.phar update $COMPOSER_FLAGS