How to get the value of a variable in PHP in HTML

How can I get the value of a variable in PHP in HTML? I have two files, one is sms. php is index.html. I generated a variable $ a in the php file. I want to get the value of $ a in html and make JS judgments.

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= I have a course or something like-I am a newbie and I am a newbie--I am decisive and don't understand

Can I merge the two files into one? include ('./index.html') is referenced at the bottom of sms. php ');
Then, the corresponding part of index.html is directly Just do it.

Can I merge the two files into one? include ('./index.html') is referenced at the bottom of sms. php ');
Then, the corresponding part of index.html is directly Just do it.
I submitted the parameter to php in index.html and made a judgment. only the variable--must be generated and the variable value must be accepted in html.