diff --git a/wp-admin/includes/noop.php b/wp-admin/includes/noop.php
index 134bef7efe..6e2397ea2e 100644
--- a/wp-admin/includes/noop.php
+++ b/wp-admin/includes/noop.php
@@ -93,13 +93,6 @@ function includes_url() {}
  */
 function wp_guess_url() {}
 
-if ( ! function_exists( 'json_encode' ) ) :
-	/**
-	 * @ignore
-	 */
-	function json_encode() {}
-endif;
-
 function get_file( $path ) {
 
 	if ( function_exists( 'realpath' ) ) {
diff --git a/wp-includes/class-wp-customize-manager.php b/wp-includes/class-wp-customize-manager.php
index 1948e7a794..2636489bea 100644
--- a/wp-includes/class-wp-customize-manager.php
+++ b/wp-includes/class-wp-customize-manager.php
@@ -1116,8 +1116,9 @@ final class WP_Customize_Manager {
 			return new WP_Error( 'wrong_post_type' );
 		}
 		$changeset_data = json_decode( $changeset_post->post_content, true );
-		if ( function_exists( 'json_last_error' ) && json_last_error() ) {
-			return new WP_Error( 'json_parse_error', '', json_last_error() );
+		$last_error     = json_last_error();
+		if ( $last_error ) {
+			return new WP_Error( 'json_parse_error', '', $last_error );
 		}
 		if ( ! is_array( $changeset_data ) ) {
 			return new WP_Error( 'expected_array' );
@@ -2843,13 +2844,9 @@ final class WP_Customize_Manager {
 		}
 
 		// Gather the data for wp_insert_post()/wp_update_post().
-		$json_options = 0;
-		if ( defined( 'JSON_UNESCAPED_SLASHES' ) ) {
-			$json_options |= JSON_UNESCAPED_SLASHES; // Introduced in PHP 5.4. This is only to improve readability as slashes needn't be escaped in storage.
-		}
-		$json_options |= JSON_PRETTY_PRINT; // Also introduced in PHP 5.4, but WP defines constant for back compat. See WP Trac #30139.
-		$post_array    = array(
-			'post_content' => wp_json_encode( $data, $json_options ),
+		$post_array = array(
+			// JSON_UNESCAPED_SLASHES is only to improve readability as slashes needn't be escaped in storage.
+			'post_content' => wp_json_encode( $data, JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT ),
 		);
 		if ( $args['title'] ) {
 			$post_array['post_title'] = $args['title'];
diff --git a/wp-includes/functions.php b/wp-includes/functions.php
index d6230bd478..c5f98e5c38 100644
--- a/wp-includes/functions.php
+++ b/wp-includes/functions.php
@@ -3721,6 +3721,7 @@ function _wp_die_process_input( $message, $title = '', $args = array() ) {
  * Encode a variable into JSON, with some sanity checks.
  *
  * @since 4.1.0
+ * @since 5.3.0 No longer handles support for PHP < 5.6.
  *
  * @param mixed $data    Variable (usually an array or object) to encode as JSON.
  * @param int   $options Optional. Options to be passed to json_encode(). Default 0.
@@ -3729,39 +3730,20 @@ function _wp_die_process_input( $message, $title = '', $args = array() ) {
  * @return string|false The JSON encoded string, or false if it cannot be encoded.
  */
 function wp_json_encode( $data, $options = 0, $depth = 512 ) {
-	/*
-	 * json_encode() has had extra params added over the years.
-	 * $options was added in 5.3, and $depth in 5.5.
-	 * We need to make sure we call it with the correct arguments.
-	 */
-	if ( version_compare( PHP_VERSION, '5.5', '>=' ) ) {
-		$args = array( $data, $options, $depth );
-	} elseif ( version_compare( PHP_VERSION, '5.3', '>=' ) ) {
-		$args = array( $data, $options );
-	} else {
-		$args = array( $data );
-	}
-
-	// Prepare the data for JSON serialization.
-	$args[0] = _wp_json_prepare_data( $data );
-
-	// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- json_encode() errors are handled after this call
-	$json = @call_user_func_array( 'json_encode', $args );
+	$json = json_encode( $data, $options, $depth );
 
 	// If json_encode() was successful, no need to do more sanity checking.
-	// ... unless we're in an old version of PHP, and json_encode() returned
-	// a string containing 'null'. Then we need to do more sanity checking.
-	if ( false !== $json && ( version_compare( PHP_VERSION, '5.5', '>=' ) || false === strpos( $json, 'null' ) ) ) {
+	if ( false !== $json ) {
 		return $json;
 	}
 
 	try {
-		$args[0] = _wp_json_sanity_check( $data, $depth );
+		$data = _wp_json_sanity_check( $data, $depth );
 	} catch ( Exception $e ) {
 		return false;
 	}
 
-	return call_user_func_array( 'json_encode', $args );
+	return json_encode( $data, $options, $depth );
 }
 
 /**
@@ -3865,48 +3847,17 @@ function _wp_json_convert_string( $string ) {
  * This supports the JsonSerializable interface for PHP 5.2-5.3 as well.
  *
  * @ignore
- * @since 4.4.0
- * @access private
+ * @since      4.4.0
+ * @deprecated 5.3.0 This function is no longer needed as support for PHP 5.2-5.3
+ *                   has been dropped.
+ * @access     private
  *
  * @param mixed $data Native representation.
  * @return bool|int|float|null|string|array Data ready for `json_encode()`.
  */
 function _wp_json_prepare_data( $data ) {
-	if ( ! defined( 'WP_JSON_SERIALIZE_COMPATIBLE' ) || WP_JSON_SERIALIZE_COMPATIBLE === false ) {
-		return $data;
-	}
-
-	switch ( gettype( $data ) ) {
-		case 'boolean':
-		case 'integer':
-		case 'double':
-		case 'string':
-		case 'NULL':
-			// These values can be passed through.
-			return $data;
-
-		case 'array':
-			// Arrays must be mapped in case they also return objects.
-			return array_map( '_wp_json_prepare_data', $data );
-
-		case 'object':
-			// If this is an incomplete object (__PHP_Incomplete_Class), bail.
-			if ( ! is_object( $data ) ) {
-				return null;
-			}
-
-			if ( $data instanceof JsonSerializable ) {
-				$data = $data->jsonSerialize();
-			} else {
-				$data = get_object_vars( $data );
-			}
-
-			// Now, pass the array (or whatever was returned from jsonSerialize through).
-			return _wp_json_prepare_data( $data );
-
-		default:
-			return null;
-	}
+	_deprecated_function( __FUNCTION__, '5.3.0' );
+	return $data;
 }
 
 /**
diff --git a/wp-includes/rest-api/class-wp-rest-request.php b/wp-includes/rest-api/class-wp-rest-request.php
index 7b2a32e618..81a5564464 100644
--- a/wp-includes/rest-api/class-wp-rest-request.php
+++ b/wp-includes/rest-api/class-wp-rest-request.php
@@ -639,21 +639,16 @@ class WP_REST_Request implements ArrayAccess {
 
 		/*
 		 * Check for a parsing error.
-		 *
-		 * Note that due to WP's JSON compatibility functions, json_last_error
-		 * might not be defined: https://core.trac.wordpress.org/ticket/27799
 		 */
-		if ( null === $params && ( ! function_exists( 'json_last_error' ) || JSON_ERROR_NONE !== json_last_error() ) ) {
+		if ( null === $params && JSON_ERROR_NONE !== json_last_error() ) {
 			// Ensure subsequent calls receive error instance.
 			$this->parsed_json = false;
 
 			$error_data = array(
-				'status' => WP_Http::BAD_REQUEST,
+				'status'             => WP_Http::BAD_REQUEST,
+				'json_error_code'    => json_last_error(),
+				'json_error_message' => json_last_error_msg(),
 			);
-			if ( function_exists( 'json_last_error' ) ) {
-				$error_data['json_error_code']    = json_last_error();
-				$error_data['json_error_message'] = json_last_error_msg();
-			}
 
 			return new WP_Error( 'rest_invalid_json', __( 'Invalid JSON body passed.' ), $error_data );
 		}
diff --git a/wp-includes/rest-api/class-wp-rest-server.php b/wp-includes/rest-api/class-wp-rest-server.php
index 43e2a33b7c..05cc240c02 100644
--- a/wp-includes/rest-api/class-wp-rest-server.php
+++ b/wp-includes/rest-api/class-wp-rest-server.php
@@ -1001,14 +1001,9 @@ class WP_REST_Server {
 	 * @return bool|string Boolean false or string error message.
 	 */
 	protected function get_json_last_error() {
-		// See https://core.trac.wordpress.org/ticket/27799.
-		if ( ! function_exists( 'json_last_error' ) ) {
-			return false;
-		}
-
 		$last_error_code = json_last_error();
 
-		if ( ( defined( 'JSON_ERROR_NONE' ) && JSON_ERROR_NONE === $last_error_code ) || empty( $last_error_code ) ) {
+		if ( JSON_ERROR_NONE === $last_error_code || empty( $last_error_code ) ) {
 			return false;
 		}
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 509b247ae4..119b65c602 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.3-alpha-46205';
+$wp_version = '5.3-alpha-46206';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.