diff --git a/src/Url/UrlTransformer.php b/src/Url/UrlTransformer.php
new file mode 100644
index 0000000..99db928
--- /dev/null
+++ b/src/Url/UrlTransformer.php
@@ -0,0 +1,32 @@
+<?php
+
+namespace WPContentSync\Url;
+
+final class UrlTransformer {
+	public function transformString( string $value, UrlMappingCollection $mappings ): string {
+		$transformed = $value;
+
+		foreach ( $mappings->all() as $mapping ) {
+			$transformed = $this->replaceMapping( $transformed, $mapping );
+		}
+
+		return $transformed;
+	}
+
+	private function replaceMapping( string $value, UrlMapping $mapping ): string {
+		$source      = $mapping->sourceUrl();
+		$destination = $mapping->destinationUrl();
+
+		$replacements = array(
+			$source                              => $destination,
+			str_replace( '&', '&amp;', $source ) => str_replace( '&', '&amp;', $destination ),
+			$this->toProtocolRelative( $source ) => $this->toProtocolRelative( $destination ),
+		);
+
+		return strtr( $value, $replacements );
+	}
+
+	private function toProtocolRelative( string $url ): string {
+		return preg_replace( '#^https?:#', '', $url ) ?? $url;
+	}
+}
diff --git a/tests/Unit/Url/UrlTransformerTest.php b/tests/Unit/Url/UrlTransformerTest.php
new file mode 100644
index 0000000..1d3f71d
--- /dev/null
+++ b/tests/Unit/Url/UrlTransformerTest.php
@@ -0,0 +1,84 @@
+<?php
+
+namespace WPContentSync\Tests\Unit\Url;
+
+use PHPUnit\Framework\TestCase;
+use WPContentSync\Url\UrlMapping;
+use WPContentSync\Url\UrlMappingCollection;
+use WPContentSync\Url\UrlTransformer;
+
+class UrlTransformerTest extends TestCase {
+	public function test_it_rewrites_plain_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'Visit https://staging.example.test/about for details.',
+			$transformer->transformString( 'Visit https://example.test/about for details.', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_html_attribute_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'<a href="https://staging.example.test/about"><img src="https://staging.example.test/image.jpg"></a>',
+			$transformer->transformString(
+				'<a href="https://example.test/about"><img src="https://example.test/image.jpg"></a>',
+				$mappings
+			)
+		);
+	}
+
+	public function test_it_rewrites_escaped_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test/path?a=1&b=2', 'https://staging.example.test/path?a=1&b=2' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test/path?a=1&amp;b=2',
+			$transformer->transformString( 'https://example.test/path?a=1&amp;b=2', $mappings )
+		);
+	}
+
+	public function test_it_rewrites_protocol_relative_urls(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'//staging.example.test/uploads/file.pdf',
+			$transformer->transformString( '//example.test/uploads/file.pdf', $mappings )
+		);
+	}
+
+	public function test_it_supports_multiple_mappings(): void {
+		$transformer = new UrlTransformer();
+		$mappings    = new UrlMappingCollection(
+			array(
+				new UrlMapping( 'https://example.test', 'https://staging.example.test' ),
+				new UrlMapping( 'https://cdn.example.test', 'https://cdn.staging.example.test' ),
+			)
+		);
+
+		self::assertSame(
+			'https://staging.example.test https://cdn.staging.example.test/image.jpg',
+			$transformer->transformString( 'https://example.test https://cdn.example.test/image.jpg', $mappings )
+		);
+	}
+}