diff --git a/pkgs/by-name/ph/phpactor/package.nix b/pkgs/by-name/ph/phpactor/package.nix
index 1e6842e12424..1ca6d5454213 100644
--- a/pkgs/by-name/ph/phpactor/package.nix
+++ b/pkgs/by-name/ph/phpactor/package.nix
@@ -21,6 +21,8 @@ php.buildComposerProject2 (finalAttrs: {
 
   nativeBuildInputs = [ installShellFiles ];
 
+  php = php.withExtensions ({ all, ... }: with all; [ mbstring ]);
+
   postInstall = ''
     installShellCompletion --cmd phpactor \
     --bash <(php $out/bin/phpactor completion bash)