diff --git a/models/PhotoMosaic.php b/models/PhotoMosaic.php
index 9d4c522..605bdb0 100644
--- a/models/PhotoMosaic.php
+++ b/models/PhotoMosaic.php
@@ -114,7 +114,7 @@ class PhotoMosaic
 		}
 
 		// Sort photos by priority and date captured.
-		usort($photos, 'self::orderPhotos');
+		usort($photos, self::orderPhotos(...));
 
 		// Three portraits?
 		if ($num_portrait === 3)