diff --git a/bin/install-wp-tests.sh b/bin/install-wp-tests.sh
index b9447efd..c913ea5e 100644
--- a/bin/install-wp-tests.sh
+++ b/bin/install-wp-tests.sh
@@ -15,7 +15,7 @@ SKIP_DB_CREATE=${6-false}
 TMPDIR=${TMPDIR-/tmp}
 TMPDIR=$(echo $TMPDIR | sed -e "s/\/$//")
 WP_TESTS_DIR=${WP_TESTS_DIR-$TMPDIR/wordpress-tests-lib}
-WP_CORE_DIR=${WP_CORE_DIR-$TMPDIR/wordpress/}
+WP_CORE_DIR=${WP_CORE_DIR-$TMPDIR/wordpress}
 
 download() {
     if [ `which curl` ]; then
@@ -25,7 +25,11 @@ download() {
     fi
 }
 
-if [[ $WP_VERSION =~ ^[0-9]+\.[0-9]+$ ]]; then
+if [[ $WP_VERSION =~ ^[0-9]+\.[0-9]+\-(beta|RC)[0-9]+$ ]]; then
+	WP_BRANCH=${WP_VERSION%\-*}
+	WP_TESTS_TAG="branches/$WP_BRANCH"
+
+elif [[ $WP_VERSION =~ ^[0-9]+\.[0-9]+$ ]]; then
 	WP_TESTS_TAG="branches/$WP_VERSION"
 elif [[ $WP_VERSION =~ [0-9]+\.[0-9]+\.[0-9]+ ]]; then
 	if [[ $WP_VERSION =~ [0-9]+\.[0-9]+\.[0] ]]; then
@@ -47,7 +51,6 @@ else
 	fi
 	WP_TESTS_TAG="tags/$LATEST_VERSION"
 fi
-
 set -ex
 
 install_wp() {
@@ -59,10 +62,10 @@ install_wp() {
 	mkdir -p $WP_CORE_DIR
 
 	if [[ $WP_VERSION == 'nightly' || $WP_VERSION == 'trunk' ]]; then
-		mkdir -p $TMPDIR/wordpress-nightly
-		download https://wordpress.org/nightly-builds/wordpress-latest.zip  $TMPDIR/wordpress-nightly/wordpress-nightly.zip
-		unzip -q $TMPDIR/wordpress-nightly/wordpress-nightly.zip -d $TMPDIR/wordpress-nightly/
-		mv $TMPDIR/wordpress-nightly/wordpress/* $WP_CORE_DIR
+		mkdir -p $TMPDIR/wordpress-trunk
+		rm -rf $TMPDIR/wordpress-trunk/*
+		svn export --quiet https://core.svn.wordpress.org/trunk $TMPDIR/wordpress-trunk/wordpress
+		mv $TMPDIR/wordpress-trunk/wordpress/* $WP_CORE_DIR
 	else
 		if [ $WP_VERSION == 'latest' ]; then
 			local ARCHIVE_NAME='latest'
@@ -89,13 +92,13 @@ install_wp() {
 		tar --strip-components=1 -zxmf $TMPDIR/wordpress.tar.gz -C $WP_CORE_DIR
 	fi
 
-	download https://raw.github.com/markoheijnen/wp-mysqli/master/db.php $WP_CORE_DIR/wp-content/db.php
+	download https://raw.githubusercontent.com/markoheijnen/wp-mysqli/master/db.php $WP_CORE_DIR/wp-content/db.php
 }
 
 install_test_suite() {
 	# portable in-place argument for both GNU sed and Mac OSX sed
 	if [[ $(uname -s) == 'Darwin' ]]; then
-		local ioption='-i .bak'
+		local ioption='-i.bak'
 	else
 		local ioption='-i'
 	fi
@@ -104,8 +107,9 @@ install_test_suite() {
 	if [ ! -d $WP_TESTS_DIR ]; then
 		# set up testing suite
 		mkdir -p $WP_TESTS_DIR
-		svn co --quiet https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/includes/ $WP_TESTS_DIR/includes
-		svn co --quiet https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/data/ $WP_TESTS_DIR/data
+		rm -rf $WP_TESTS_DIR/{includes,data}
+		svn export --quiet --ignore-externals https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/includes/ $WP_TESTS_DIR/includes
+		svn export --quiet --ignore-externals https://develop.svn.wordpress.org/${WP_TESTS_TAG}/tests/phpunit/data/ $WP_TESTS_DIR/data
 	fi
 
 	if [ ! -f wp-tests-config.php ]; then
@@ -113,6 +117,7 @@ install_test_suite() {
 		# remove all forward slashes in the end
 		WP_CORE_DIR=$(echo $WP_CORE_DIR | sed "s:/\+$::")
 		sed $ioption "s:dirname( __FILE__ ) . '/src/':'$WP_CORE_DIR/':" "$WP_TESTS_DIR"/wp-tests-config.php
+		sed $ioption "s:__DIR__ . '/src/':'$WP_CORE_DIR/':" "$WP_TESTS_DIR"/wp-tests-config.php
 		sed $ioption "s/youremptytestdbnamehere/$DB_NAME/" "$WP_TESTS_DIR"/wp-tests-config.php
 		sed $ioption "s/yourusernamehere/$DB_USER/" "$WP_TESTS_DIR"/wp-tests-config.php
 		sed $ioption "s/yourpasswordhere/$DB_PASS/" "$WP_TESTS_DIR"/wp-tests-config.php
@@ -132,6 +137,23 @@ install_test_suite() {
 
 }
 
+recreate_db() {
+	shopt -s nocasematch
+	if [[ $1 =~ ^(y|yes)$ ]]
+	then
+		mysqladmin drop $DB_NAME -f --user="$DB_USER" --password="$DB_PASS"$EXTRA
+		create_db
+		echo "Recreated the database ($DB_NAME)."
+	else
+		echo "Leaving the existing database ($DB_NAME) in place."
+	fi
+	shopt -u nocasematch
+}
+
+create_db() {
+	mysqladmin create $DB_NAME --user="$DB_USER" --password="$DB_PASS"$EXTRA
+}
+
 install_db() {
 
 	if [ ${SKIP_DB_CREATE} = "true" ]; then
@@ -155,7 +177,14 @@ install_db() {
 	fi
 
 	# create database
-	mysqladmin create $DB_NAME --user="$DB_USER" --password="$DB_PASS"$EXTRA
+	if [ $(mysql --user="$DB_USER" --password="$DB_PASS"$EXTRA --execute='show databases;' | grep ^$DB_NAME$) ]
+	then
+		echo "Reinstalling will delete the existing test database ($DB_NAME)"
+		read -p 'Are you sure you want to proceed? [y/N]: ' DELETE_EXISTING_DB
+		recreate_db $DELETE_EXISTING_DB
+	else
+		create_db
+	fi
 }
 
 install_wp
diff --git a/tests/bootstrap.php b/tests/bootstrap.php
index 2a307196..4e5991f7 100644
--- a/tests/bootstrap.php
+++ b/tests/bootstrap.php
@@ -1,6 +1,6 @@
 <?php
 /**
- * PHPUnit bootstrap file
+ * PHPUnit bootstrap file.
  */
 
 $_tests_dir = getenv( 'WP_TESTS_DIR' );
@@ -9,13 +9,19 @@
 	$_tests_dir = rtrim( sys_get_temp_dir(), '/\\' ) . '/wordpress-tests-lib';
 }
 
-if ( ! file_exists( $_tests_dir . '/includes/functions.php' ) ) {
-	echo "Could not find $_tests_dir/includes/functions.php, have you run bin/install-wp-tests.sh ?";
+// Forward custom PHPUnit Polyfills configuration to PHPUnit bootstrap file.
+$_phpunit_polyfills_path = getenv( 'WP_TESTS_PHPUNIT_POLYFILLS_PATH' );
+if ( false !== $_phpunit_polyfills_path ) {
+	define( 'WP_TESTS_PHPUNIT_POLYFILLS_PATH', $_phpunit_polyfills_path );
+}
+
+if ( ! file_exists( "{$_tests_dir}/includes/functions.php" ) ) {
+	echo "Could not find {$_tests_dir}/includes/functions.php, have you run bin/install-wp-tests.sh ?" . PHP_EOL; // phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped
 	exit( 1 );
 }
 
 // Give access to tests_add_filter() function.
-require_once $_tests_dir . '/includes/functions.php';
+require_once "{$_tests_dir}/includes/functions.php";
 
 /**
  * Manually load the plugin being tested.
@@ -27,4 +33,4 @@ function _manually_load_plugin() {
 tests_add_filter( 'muplugins_loaded', '_manually_load_plugin' );
 
 // Start up the WP testing environment.
-require $_tests_dir . '/includes/bootstrap.php';
+require "{$_tests_dir}/includes/bootstrap.php";