How to add class name to an existing tag in php

Question

I have a hidden tag in html code:

<p class="class1">something<p>

and php code:

if(SomeCondition){
 select element by DOM and add class "active" so that element will be shown
}

how can I select element by DOM specifically in this php file and perform operations on an element, for example add class or remove class or change its properties (f.e.: bg color: black). Thank you.

P.S. SomeCondition is server-sided

Answer 1

You don't do it in PHP that way. I mean not in a simple way. This is done in JS / jQuery, e.g.

$(document).ready(function(){  
   $(".class1").addClass("active"); 
});