Why is an error reported when I define a string "<kph>" in the following way?

I use the following method to define a String & lt; kph & gt;. why is an error reported? I use the following method to define a String" "Why is an error reported? If it is changed to "123", it will be okay. why?
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$ Aay = array (" ", '12 ');
Echo $ aay [0];
?>

Reply to discussion (solution)

Who reported the error? Error message

The following error message is displayed:

HTML1521: invalid "" or the end of the file. All elements with the start mark should end correctly before the end of the document.
File: ss. php, row: 4, column: 6

It's just a false positive in your editor.

I used the IE browser for debugging and running. this error report is not an editor report.

This is not a php error.
You printed the string in php. The tag is parsed by the browser.
It is normal for browsers like IE in ancient times to report an error if they cannot find the closed tag.
Google will not report any errors in the new age.
So change the browser first, or do not output This special content