phpunit/src/SsaAnalysisTest.php

@@ -1263,6 +1263,15 @@ class SsaAnalysisTest extends TestCase
         }
     }
 
+    public function testDetectTypeOfConcatExpressions(): void
+    {
+        $concat = new Expr\BinaryOp\Concat(new Scalar\LNumber(1), new Scalar\String_(''));
+        $concatAssign = new Expr\AssignOp\Concat(new Expr\Variable('value'), new Scalar\LNumber(2));
+
+        $this->assertSame(Type::STR, $this->invoke('detectTypeOfExpr', $concat));
+        $this->assertSame(Type::STR, $this->invoke('detectTypeOfExpr', $concatAssign));
+    }
+
     public function testDetectSsaDefTypeNull(): void
     {
         $ssaVar = new SsaVar(1, 'x');

src/CompilerBase.php

@@ -2299,6 +2299,9 @@ class CompilerBase implements PropertyAccessContext
             case 'Expr_Array':
             case 'Expr_Cast_Array':
                 return Type::ARRAY;
+            case 'Expr_BinaryOp_Concat':
+            case 'Expr_AssignOp_Concat':
+                return Type::STR;
             case 'Expr_BinaryOp_Plus':
             case 'Expr_BinaryOp_Minus':
             case 'Expr_BinaryOp_Mul':

src/Parser/BinaryOpTrait.php

@@ -265,6 +265,13 @@ trait BinaryOpTrait
 
         $argList = $prefixExpressions;
         foreach ($items as $item) {
+            // Keep one operand so concat still performs PHP string coercion.
+            // Prefix expressions are operands too (for example, the left-hand
+            // value of `.=`), so an empty RHS literal can be omitted there.
+            if ($argList !== [] && $this->isScalarString($item) && $item->value === '') {
+                continue;
+            }
+
             $type = $this->detectTypeOfExpr($item);
             $parsed = $this->parseExprAsValue($item);
             $argList[] = $this->prepareConcatOperand($parsed, $type);

tests/compiler/concat_empty.phpt

@@ -0,0 +1,33 @@
+--TEST--
+Concat empty strings
+--FILE--
+<?php
+function main() {
+    var_dump('' . 1 . '' . 'a' . '');
+    var_dump('' . 1);
+    var_dump(1 . '');
+    var_dump(false . '');
+    var_dump('' . '' . '');
+
+    $value = 'value';
+    var_dump('' . $value . '');
+    var_dump('' . get_value() . '');
+
+    $number = 1;
+    $number .= '';
+    var_dump($number);
+}
+
+function get_value(): string {
+    return 'called';
+}
+?>
+--EXPECT--
+string(2) "1a"
+string(1) "1"
+string(1) "1"
+string(0) ""
+string(0) ""
+string(5) "value"
+string(6) "called"
+string(1) "1"