Source

File: wp-includes/rest-api/class-wp-rest-server.php

public static function get_raw_data() {
		// phpcs:disable PHPCompatibility.Variables.RemovedPredefinedGlobalVariables.http_raw_post_dataDeprecatedRemoved
		global $HTTP_RAW_POST_DATA;

		// $HTTP_RAW_POST_DATA was deprecated in PHP 5.6 and removed in PHP 7.0.
		if ( ! isset( $HTTP_RAW_POST_DATA ) ) {
			$HTTP_RAW_POST_DATA = file_get_contents( 'php://input' );
		}

		return $HTTP_RAW_POST_DATA;
		// phpcs:enable
	}