diff --git a/internal/extgen/classparser.go b/internal/extgen/classparser.go
index caef0ea239..36f7751158 100644
--- a/internal/extgen/classparser.go
+++ b/internal/extgen/classparser.go
@@ -86,7 +86,7 @@ func (cp *classParser) parse(filename string) (classes []phpClass, err error) {
 			}
 
 			if err := validator.validateClass(class); err != nil {
-				fmt.Printf("Warning: Invalid class '%s': %v\n", class.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Invalid class '%s': %v\n", class.Name, err)
 				continue
 			}
 
@@ -231,7 +231,7 @@ func (cp *classParser) parseMethods(filename string) (methods []phpClassMethod,
 
 			method, err := cp.parseMethodSignature(className, signature)
 			if err != nil {
-				fmt.Printf("Warning: Error parsing method signature %q: %v\n", signature, err)
+				fmt.Fprintf(os.Stderr, "Warning: Error parsing method signature %q: %v\n", signature, err)
 
 				continue
 			}
@@ -246,7 +246,7 @@ func (cp *classParser) parseMethods(filename string) (methods []phpClassMethod,
 			}
 
 			if err := validator.validateTypes(phpFunc); err != nil {
-				fmt.Printf("Warning: Method \"%s::%s\" uses unsupported types: %v\n", className, method.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Method \"%s::%s\" uses unsupported types: %v\n", className, method.Name, err)
 
 				continue
 			}
@@ -274,7 +274,7 @@ func (cp *classParser) parseMethods(filename string) (methods []phpClassMethod,
 			}
 
 			if err := validator.validateGoFunctionSignatureWithOptions(phpFunc, true); err != nil {
-				fmt.Printf("Warning: Go method signature mismatch for '%s::%s': %v\n", currentMethod.ClassName, currentMethod.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Go method signature mismatch for '%s::%s': %v\n", currentMethod.ClassName, currentMethod.Name, err)
 				currentMethod = nil
 				continue
 			}
diff --git a/internal/extgen/funcparser.go b/internal/extgen/funcparser.go
index 5e64de1b4c..207c0a6c4a 100644
--- a/internal/extgen/funcparser.go
+++ b/internal/extgen/funcparser.go
@@ -39,19 +39,19 @@ func (fp *FuncParser) parse(filename string) (functions []phpFunction, err error
 			signature := strings.TrimSpace(matches[1])
 			phpFunc, err := fp.parseSignature(signature)
 			if err != nil {
-				fmt.Printf("Warning: Error parsing signature '%s': %v\n", signature, err)
+				fmt.Fprintf(os.Stderr, "Warning: Error parsing signature '%s': %v\n", signature, err)
 
 				continue
 			}
 
 			if err := validator.validateFunction(*phpFunc); err != nil {
-				fmt.Printf("Warning: Invalid function '%s': %v\n", phpFunc.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Invalid function '%s': %v\n", phpFunc.Name, err)
 
 				continue
 			}
 
 			if err := validator.validateTypes(*phpFunc); err != nil {
-				fmt.Printf("Warning: Function '%s' uses unsupported types: %v\n", phpFunc.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Function '%s' uses unsupported types: %v\n", phpFunc.Name, err)
 
 				continue
 			}
@@ -69,7 +69,7 @@ func (fp *FuncParser) parse(filename string) (functions []phpFunction, err error
 			currentPHPFunc.GoFunction = goFunc
 
 			if err := validator.validateGoFunctionSignatureWithOptions(*currentPHPFunc, false); err != nil {
-				fmt.Printf("Warning: Go function signature mismatch for %q: %v\n", currentPHPFunc.Name, err)
+				fmt.Fprintf(os.Stderr, "Warning: Go function signature mismatch for %q: %v\n", currentPHPFunc.Name, err)
 				currentPHPFunc = nil
 
 				continue