diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index 7d71c8c56963d..7a8abb2b6b176 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -6954,8 +6954,12 @@ function get_file_data( $file, $default_headers, $context = '' ) {
 		$file_data = '';
 	}
 
-	// Make sure we catch CR-only line endings.
-	$file_data = str_replace( "\r", "\n", $file_data );
+	// Make sure we catch CR-only line endings and strip PHP open/close tags.
+	$file_data = str_replace(
+		array( "\r", '<?php', '<?', '?>' ),
+		array( "\n", '', '', '' ),
+		$file_data
+	);
 
 	/**
 	 * Filters extra file headers by context.
diff --git a/tests/phpunit/data/formatting/file-header-short-open-tag.php b/tests/phpunit/data/formatting/file-header-short-open-tag.php
new file mode 100644
index 0000000000000..b2353ea6a4502
--- /dev/null
+++ b/tests/phpunit/data/formatting/file-header-short-open-tag.php
@@ -0,0 +1 @@
+<? # Template Name: Short Tag Template ?>
diff --git a/tests/phpunit/tests/file.php b/tests/phpunit/tests/file.php
index 9a29e4e67669a..1f416a8a74b64 100644
--- a/tests/phpunit/tests/file.php
+++ b/tests/phpunit/tests/file.php
@@ -95,6 +95,30 @@ public function test_get_file_data_with_php_open_tag_prefix() {
 		}
 	}
 
+	/**
+	 * Tests that get_file_data() supports single-line headers with a <?php open tag.
+	 *
+	 * @ticket 42517
+	 * @covers ::get_file_data
+	 */
+	public function test_get_file_data_with_single_line_php_open_tag() {
+		$headers = array( 'TemplateName' => 'Template Name' );
+		$actual  = get_file_data( DIR_TESTDATA . '/themedir1/page-templates/template-header.php', $headers );
+		$this->assertSame( 'This Template Header Is On One Line', $actual['TemplateName'] );
+	}
+
+	/**
+	 * Tests that get_file_data() supports single-line headers with a <? short open tag.
+	 *
+	 * @ticket 42517
+	 * @covers ::get_file_data
+	 */
+	public function test_get_file_data_with_single_line_short_open_tag() {
+		$headers = array( 'TemplateName' => 'Template Name' );
+		$actual  = get_file_data( DIR_TESTDATA . '/formatting/file-header-short-open-tag.php', $headers );
+		$this->assertSame( 'Short Tag Template', $actual['TemplateName'] );
+	}
+
 	private function is_unique_writable_file( $path, $filename ) {
 		$fullpath = $path . DIRECTORY_SEPARATOR . $filename;