PHP newline problem

Question

I encountered a problem while reading PHP-related tutorial books. Please forgive me for being too clumsy. Forgive me for using a capitalized book title number instead of <>, otherwise a line break will be automatically displayed.
echo "a: [".(20>9)."]《br》";
The above is a simple Boolean expression
Why should the line break symbol of "br" be written in ""? And if it is written between "and;, an error will be reported?
(

Answer 1

你要明白為什麼寫在""雙引號內，由於寫起雙引號內，編譯才會視<br>為字符串輸出出來。但在引號外,<br>不是變量和常量，所以就是出現報錯！

Answer 2

You can echo "a:[.(20>9)."], '<br>';