diff --git a/src/quick-lint-js/diag/diagnostic-types-2.h b/src/quick-lint-js/diag/diagnostic-types-2.h
index 49b7a9059a..e299f347a2 100644
--- a/src/quick-lint-js/diag/diagnostic-types-2.h
+++ b/src/quick-lint-js/diag/diagnostic-types-2.h
@@ -32,6 +32,13 @@ QLJS_RESERVED_DIAG("E0279")
 QLJS_RESERVED_DIAG("E0391")
 QLJS_RESERVED_DIAG("E0707")
 
+struct Diag_EJS_or_PHP_Syntax_Detected {
+    [[qljs::diag("E0991", Diagnostic_Severity::warning)]]  //
+    [[qljs::message("Detected potential EJS or PHP syntax",
+                  ARG(span))]]  //
+    Source_Code_Span span;
+};
+
 struct Diag_Abstract_Field_Cannot_Have_Initializer {
   [[qljs::diag("E0295", Diagnostic_Severity::error)]]  //
   [[qljs::message("abstract fields cannot have default values",
diff --git a/src/quick-lint-js/fe/lex.cpp b/src/quick-lint-js/fe/lex.cpp
index 565f1e6e02..05407652b5 100644
--- a/src/quick-lint-js/fe/lex.cpp
+++ b/src/quick-lint-js/fe/lex.cpp
@@ -432,8 +432,16 @@ bool Lexer::try_parse_current_token() {
     break;
 
   case '<':
-    if (this->input_[1] == '!' && this->input_[2] == '-' &&
-        this->input_[3] == '-') {
+  // checking for EJS or PHP syntax
+    if((this->input_[0] == '<' && (this->input_[1] == '%')) || (this->input_[0] == '?')){
+      this->diag_reporter_->report(Diag_EJS_or_PHP_Syntax_Detected{
+        Source_Code_Span(&this->input_[0], &this->input_[2])
+      });
+      // moving the input pointer after detecting sequence
+      this->input_+=2;
+      return false;
+    }else if (this->input_[1] == '!' && this->input_[2] == '-' &&
+      this->input_[3] == '-') {
       this->input_ += 4;
       this->skip_line_comment_body();
       return false;
diff --git a/test/test-parse-warning.cpp b/test/test-parse-warning.cpp
index bad6c9877d..0c2976fe31 100644
--- a/test/test-parse-warning.cpp
+++ b/test/test-parse-warning.cpp
@@ -31,6 +31,11 @@ class Test_Parse_Warning : public Test_Parse_Expression {};
 class Test_Error_Equals_Does_Not_Distribute_Over_Or
     : public Test_Parse_Expression {};
 
+TEST_F(Test_Parse_Warning, warn_on_ejs_or_php_syntax) {
+    test_lex("window.audioRecordingBitRate = <%= audioRecordingBitRate %>;",
+             "  ^^^ Diag_EJS_or_PHP_Syntax_Detected"_diag);
+}
+
 TEST_F(Test_Parse_Warning, condition_with_assignment_from_literal) {
   {
     Spy_Visitor p = test_parse_and_visit_statement(