Index: src/wp-includes/load.php
===================================================================
--- src/wp-includes/load.php	(revision 40893)
+++ src/wp-includes/load.php	(working copy)
@@ -325,9 +325,17 @@
 		elseif ( null !== WP_DEBUG_DISPLAY )
 			ini_set( 'display_errors', 0 );
 
-		if ( WP_DEBUG_LOG ) {
+		if ( in_array( WP_DEBUG_LOG, array( true, 'true', 'TRUE', 1, '1' ), true ) ) {
+			$log_path = WP_CONTENT_DIR . '/debug.log';
+		} elseif ( is_string( WP_DEBUG_LOG ) && 0 === validate_file( WP_DEBUG_LOG ) ) {
+			$log_path = WP_DEBUG_LOG;
+		} else {
+			$log_path = false;
+		}
+
+		if ( false !== $log_path && is_writeable( $log_path ) ) {
 			ini_set( 'log_errors', 1 );
-			ini_set( 'error_log', WP_CONTENT_DIR . '/debug.log' );
+			ini_set( 'error_log', $log_path );
 		}
 	} else {
 		error_reporting( E_CORE_ERROR | E_CORE_WARNING | E_COMPILE_ERROR | E_ERROR | E_WARNING | E_PARSE | E_USER_ERROR | E_USER_WARNING | E_RECOVERABLE_ERROR );