new inspection for string nullability

src/linter/quickfix.go

@@ -47,6 +47,14 @@ func (g *QuickFixGenerator) NullForNotNullableProperty(prop *ir.PropertyStmt) qu
 	}
 }
 
+func (g *QuickFixGenerator) NullableStringType(param *ir.Name) quickfix.TextEdit {
+	return quickfix.TextEdit{
+		StartPos:    param.Position.StartPos,
+		EndPos:      param.Position.EndPos,
+		Replacement: "?string",
+	}
+}
+
 func (g *QuickFixGenerator) GetType(node ir.Node, isFunctionName, nodeText string, isNegative bool) quickfix.TextEdit {
 	pos := ir.GetPosition(node)
 

src/linter/report.go

@@ -25,6 +25,15 @@ func addBuiltinCheckers(reg *CheckersRegistry) {
 			After:    `$s = strip_tags($s, '<br>')`,
 		},
 
+		{
+			Name:     "nullableString",
+			Default:  true,
+			Quickfix: true,
+			Comment:  "Report not nullable string can be null.",
+			Before:   `function f(string $str = null);`,
+			After:    `function f(?string $str = null);`,
+		},
+
 		{
 			Name:     "emptyStmt",
 			Default:  true,

src/linter/root_checker.go

@@ -604,8 +604,24 @@ func (r *rootChecker) checkFuncParam(p *ir.Parameter) {
 		}
 		return true
 	})
-
 	r.CheckTypeHintFunctionParam(p)
+	r.CheckParamStringNullability(p)
+}
+
+func (r *rootChecker) CheckParamStringNullability(p *ir.Parameter) {
+	if param, ok := p.VariableType.(*ir.Name); ok {
+		if param.Value != "string" {
+			return
+		}
+
+		if defValue, ok := p.DefaultValue.(*ir.ConstFetchExpr); ok {
+			if defValue.Constant.Value != "null" {
+				return
+			}
+			r.walker.Report(param, LevelWarning, "nullableString", "string with null default value should be explicitly nullable")
+			r.walker.addQuickFix("nullableString", r.quickfix.NullableStringType(param))
+		}
+	}
 }
 
 func (r *rootChecker) CheckTypeHintFunctionParam(p *ir.Parameter) {

src/tests/golden/testdata/quickfix/stringNullable.fix.expected

@@ -0,0 +1,5 @@
+<?php
+
+function f(?string $filed = null) {
+return 1;
+}

src/tests/golden/testdata/quickfix/stringNullable.php

@@ -0,0 +1,5 @@
+<?php
+
+function f(string $filed = null) {
+return 1;
+}