How to reference a variable in PHP in JavaScript

How do I refer to a variable in PHP in JavaScript?
For example: In PHP already has a variable $filename value of "1.txt", the user click the Delete button to pop up the JavaScript confirmation dialog box, the content is "delete 1.txt". How do I reference the value of $filename?
(Beginner, problem may be some idiot, please answer patiently, thank you)

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How do I refer to a variable in PHP in JavaScript?
For example: In PHP already has a variable $filename value of "1.txt", the user click the Delete button to pop up the JavaScript confirmation dialog box, the content is "delete 1.txt". How do I reference the value of $filename?
(Beginner, problem may be some idiot, please answer patiently, thank you)

var name = "
  
  ";$('#button').click(function(){    alert(name);})

https://segmentfault.com/q/1010000005145137

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