diff --git a/wp-admin/includes/ajax-actions.php b/wp-admin/includes/ajax-actions.php
index 35090e2288..a4e4e810df 100644
--- a/wp-admin/includes/ajax-actions.php
+++ b/wp-admin/includes/ajax-actions.php
@@ -2532,7 +2532,7 @@ function wp_ajax_upload_attachment() {
 	check_ajax_referer( 'media-form' );
 	/*
 	 * This function does not use wp_send_json_success() / wp_send_json_error()
-	 * as the html4 Plupload handler requires a text/html content-type for older IE.
+	 * as the html4 Plupload handler requires a text/html Content-Type for older IE.
 	 * See https://core.trac.wordpress.org/ticket/31037
 	 */
 
diff --git a/wp-admin/includes/file.php b/wp-admin/includes/file.php
index 62e9c0cb91..6d22898b9c 100644
--- a/wp-admin/includes/file.php
+++ b/wp-admin/includes/file.php
@@ -1184,7 +1184,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 		return new WP_Error( 'http_404', trim( wp_remote_retrieve_response_message( $response ) ), $data );
 	}
 
-	$content_disposition = wp_remote_retrieve_header( $response, 'content-disposition' );
+	$content_disposition = wp_remote_retrieve_header( $response, 'Content-Disposition' );
 
 	if ( $content_disposition ) {
 		$content_disposition = strtolower( $content_disposition );
@@ -1211,7 +1211,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 		}
 	}
 
-	$content_md5 = wp_remote_retrieve_header( $response, 'content-md5' );
+	$content_md5 = wp_remote_retrieve_header( $response, 'Content-MD5' );
 
 	if ( $content_md5 ) {
 		$md5_check = verify_file_md5( $tmpfname, $content_md5 );
@@ -1238,7 +1238,7 @@ function download_url( $url, $timeout = 300, $signature_verification = false ) {
 
 	// Perform signature valiation if supported.
 	if ( $signature_verification ) {
-		$signature = wp_remote_retrieve_header( $response, 'x-content-signature' );
+		$signature = wp_remote_retrieve_header( $response, 'X-Content-Signature' );
 
 		if ( ! $signature ) {
 			// Retrieve signatures from a file if the header wasn't included.
diff --git a/wp-admin/includes/upgrade.php b/wp-admin/includes/upgrade.php
index 63580cceb0..9c4cb43580 100644
--- a/wp-admin/includes/upgrade.php
+++ b/wp-admin/includes/upgrade.php
@@ -512,7 +512,7 @@ function wp_install_maybe_enable_pretty_permalinks() {
 		 * can block head requests.
 		 */
 		$response          = wp_remote_get( $test_url, array( 'timeout' => 5 ) );
-		$x_pingback_header = wp_remote_retrieve_header( $response, 'x-pingback' );
+		$x_pingback_header = wp_remote_retrieve_header( $response, 'X-Pingback' );
 		$pretty_permalinks = $x_pingback_header && get_bloginfo( 'pingback_url' ) === $x_pingback_header;
 
 		if ( $pretty_permalinks ) {
diff --git a/wp-includes/class-json.php b/wp-includes/class-json.php
index d651dcdfb8..6091d567f5 100644
--- a/wp-includes/class-json.php
+++ b/wp-includes/class-json.php
@@ -291,7 +291,7 @@ class Services_JSON
     {
         _deprecated_function( __METHOD__, '5.3.0', 'The PHP native JSON extension' );
 
-        header('Content-type: application/json');
+        header('Content-Type: application/json');
         return $this->encodeUnsafe($var);
     }
     /**
diff --git a/wp-includes/class-simplepie.php b/wp-includes/class-simplepie.php
index a392e54cd9..3d639fc709 100644
--- a/wp-includes/class-simplepie.php
+++ b/wp-includes/class-simplepie.php
@@ -1864,7 +1864,7 @@ class SimplePie
 	}
 
 	/**
-	 * Send the content-type header with correct encoding
+	 * Send the Content-Type header with correct encoding
 	 *
 	 * This method ensures that the SimplePie-enabled page is being served with
 	 * the correct {@link http://www.iana.org/assignments/media-types/ mime-type}
@@ -1886,7 +1886,7 @@ class SimplePie
 	{
 		if (!headers_sent())
 		{
-			$header = "Content-type: $mime;";
+			$header = "Content-Type: $mime;";
 			if ($this->get_encoding())
 			{
 				$header .= ' charset=' . $this->get_encoding();
diff --git a/wp-includes/class-snoopy.php b/wp-includes/class-snoopy.php
index fb6857c525..e82c673989 100644
--- a/wp-includes/class-snoopy.php
+++ b/wp-includes/class-snoopy.php
@@ -53,7 +53,7 @@ class Snoopy
 	var $cookies		=	array();			// array of cookies to pass
 												// $cookies["username"]="joe";
 	var	$rawheaders		=	array();			// array of raw headers to send
-												// $rawheaders["Content-type"]="text/html";
+												// $rawheaders["Content-Type"]="text/html";
 
 	var $maxredirs		=	5;					// http redirection depth maximum. 0 = disallow
 	var $lastredirectaddr	=	"";				// contains address of last redirected address
@@ -825,13 +825,13 @@ class Snoopy
 				$headers .= $headerKey.": ".$headerVal."\r\n";
 		}
 		if(!empty($content_type)) {
-			$headers .= "Content-type: $content_type";
+			$headers .= "Content-Type: $content_type";
 			if ($content_type == "multipart/form-data")
 				$headers .= "; boundary=".$this->_mime_boundary;
 			$headers .= "\r\n";
 		}
 		if(!empty($body))
-			$headers .= "Content-length: ".strlen($body)."\r\n";
+			$headers .= "Content-Length: ".strlen($body)."\r\n";
 		if(!empty($this->user) || !empty($this->pass))
 			$headers .= "Authorization: Basic ".base64_encode($this->user.":".$this->pass)."\r\n";
 
@@ -990,12 +990,12 @@ class Snoopy
 		}
 		if(!empty($content_type)) {
 			if ($content_type == "multipart/form-data")
-				$headers[] = "Content-type: $content_type; boundary=".$this->_mime_boundary;
+				$headers[] = "Content-Type: $content_type; boundary=".$this->_mime_boundary;
 			else
-				$headers[] = "Content-type: $content_type";
+				$headers[] = "Content-Type: $content_type";
 		}
 		if(!empty($body))
-			$headers[] = "Content-length: ".strlen($body);
+			$headers[] = "Content-Length: ".strlen($body);
 		if(!empty($this->user) || !empty($this->pass))
 			$headers[] = "Authorization: BASIC ".base64_encode($this->user.":".$this->pass);
 
diff --git a/wp-includes/comment.php b/wp-includes/comment.php
index a52edf84f4..50a761f94a 100644
--- a/wp-includes/comment.php
+++ b/wp-includes/comment.php
@@ -2824,12 +2824,12 @@ function discover_pingback_server_uri( $url, $deprecated = '' ) {
 		return false;
 	}
 
-	if ( wp_remote_retrieve_header( $response, 'x-pingback' ) ) {
-		return wp_remote_retrieve_header( $response, 'x-pingback' );
+	if ( wp_remote_retrieve_header( $response, 'X-Pingback' ) ) {
+		return wp_remote_retrieve_header( $response, 'X-Pingback' );
 	}
 
 	// Not an (x)html, sgml, or xml page, no use going further.
-	if ( preg_match( '#(image|audio|video|model)/#is', wp_remote_retrieve_header( $response, 'content-type' ) ) ) {
+	if ( preg_match( '#(image|audio|video|model)/#is', wp_remote_retrieve_header( $response, 'Content-Type' ) ) ) {
 		return false;
 	}
 
diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index 623a69aba0..1d285b6cf7 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -943,8 +943,8 @@ function do_enclose( $content, $post ) {
 
 			$headers = wp_get_http_headers( $url );
 			if ( $headers ) {
-				$len           = isset( $headers['content-length'] ) ? (int) $headers['content-length'] : 0;
-				$type          = isset( $headers['content-type'] ) ? $headers['content-type'] : '';
+				$len           = isset( $headers['Content-Length'] ) ? (int) $headers['Content-Length'] : 0;
+				$type          = isset( $headers['Content-Type'] ) ? $headers['Content-Type'] : '';
 				$allowed_types = array( 'video', 'audio' );
 
 				// Check to see if we can figure out the mime type from the extension.
diff --git a/wp-includes/general-template.php b/wp-includes/general-template.php
index ba5e89737c..5f234f47a2 100644
--- a/wp-includes/general-template.php
+++ b/wp-includes/general-template.php
@@ -755,7 +755,7 @@ function bloginfo( $show = '' ) {
  * - 'admin_email' - Admin email (set in Settings > General)
  * - 'charset' - The "Encoding for pages and feeds"  (set in Settings > Reading)
  * - 'version' - The current WordPress version
- * - 'html_type' - The content-type (default: "text/html"). Themes and plugins
+ * - 'html_type' - The Content-Type (default: "text/html"). Themes and plugins
  *   can override the default value using the {@see 'pre_option_html_type'} filter
  * - 'text_direction' - The text direction determined by the site's language. is_rtl()
  *   should be used instead
diff --git a/wp-includes/pluggable.php b/wp-includes/pluggable.php
index a3cb0408b4..dded2fee86 100644
--- a/wp-includes/pluggable.php
+++ b/wp-includes/pluggable.php
@@ -463,7 +463,7 @@ if ( ! function_exists( 'wp_mail' ) ) :
 
 		// Set Content-Type and charset.
 
-		// If we don't have a content-type from the input headers.
+		// If we don't have a Content-Type from the input headers.
 		if ( ! isset( $content_type ) ) {
 			$content_type = 'text/plain';
 		}
diff --git a/wp-includes/rest-api/class-wp-rest-request.php b/wp-includes/rest-api/class-wp-rest-request.php
index 494abd2360..18e867de3a 100644
--- a/wp-includes/rest-api/class-wp-rest-request.php
+++ b/wp-includes/rest-api/class-wp-rest-request.php
@@ -291,16 +291,16 @@ class WP_REST_Request implements ArrayAccess {
 	}
 
 	/**
-	 * Retrieves the content-type of the request.
+	 * Retrieves the Content-Type of the request.
 	 *
 	 * @since 4.4.0
 	 *
 	 * @return array|null Map containing 'value' and 'parameters' keys
-	 *                    or null when no valid content-type header was
+	 *                    or null when no valid Content-Type header was
 	 *                    available.
 	 */
 	public function get_content_type() {
-		$value = $this->get_header( 'content-type' );
+		$value = $this->get_header( 'Content-Type' );
 		if ( empty( $value ) ) {
 			return null;
 		}
@@ -325,11 +325,11 @@ class WP_REST_Request implements ArrayAccess {
 	}
 
 	/**
-	 * Checks if the request has specified a JSON content-type.
+	 * Checks if the request has specified a JSON Content-Type.
 	 *
 	 * @since 5.6.0
 	 *
-	 * @return bool True if the content-type header is JSON.
+	 * @return bool True if the Content-Type header is JSON.
 	 */
 	public function is_json_content_type() {
 		$content_type = $this->get_content_type();
@@ -719,7 +719,7 @@ class WP_REST_Request implements ArrayAccess {
 		$this->parsed_body = true;
 
 		/*
-		 * Check that we got URL-encoded. Treat a missing content-type as
+		 * Check that we got URL-encoded. Treat a missing Content-Type as
 		 * URL-encoded for maximum compatibility.
 		 */
 		$content_type = $this->get_content_type();
diff --git a/wp-includes/sitemaps/class-wp-sitemaps-renderer.php b/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
index 11d5c3820a..9f91a10b7c 100644
--- a/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
+++ b/wp-includes/sitemaps/class-wp-sitemaps-renderer.php
@@ -123,7 +123,7 @@ class WP_Sitemaps_Renderer {
 	 * @param array $sitemaps Array of sitemap URLs.
 	 */
 	public function render_index( $sitemaps ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		$this->check_for_simple_xml_availability();
 
@@ -188,7 +188,7 @@ class WP_Sitemaps_Renderer {
 	 * @param array $url_list Array of URLs for a sitemap.
 	 */
 	public function render_sitemap( $url_list ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		$this->check_for_simple_xml_availability();
 
diff --git a/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php b/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
index 8268fcc924..f752a7f55f 100644
--- a/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
+++ b/wp-includes/sitemaps/class-wp-sitemaps-stylesheet.php
@@ -22,7 +22,7 @@ class WP_Sitemaps_Stylesheet {
 	 * @param string $type Stylesheet type. Either 'sitemap' or 'index'.
 	 */
 	public function render_stylesheet( $type ) {
-		header( 'Content-type: application/xml; charset=UTF-8' );
+		header( 'Content-Type: application/xml; charset=UTF-8' );
 
 		if ( 'sitemap' === $type ) {
 			// phpcs:ignore WordPress.Security.EscapeOutput.OutputNotEscaped -- All content escaped below.
diff --git a/wp-includes/version.php b/wp-includes/version.php
index dd72204902..12301dbaea 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.2-alpha-55209';
+$wp_version = '6.2-alpha-55210';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.
diff --git a/wp-mail.php b/wp-mail.php
index 631645cbed..2600521e38 100644
--- a/wp-mail.php
+++ b/wp-mail.php
@@ -171,7 +171,7 @@ for ( $i = 1; $i <= $count; $i++ ) {
 		$content = explode( '--' . $boundary, $content );
 		$content = $content[2];
 
-		// Match case-insensitive content-transfer-encoding.
+		// Match case-insensitive Content-Transfer-Encoding.
 		if ( preg_match( '/Content-Transfer-Encoding: quoted-printable/i', $content, $delim ) ) {
 			$content = explode( $delim[0], $content );
 			$content = $content[1];