link to file path name..

I have a PHP file that involved with codemirror which works great, but I want each text area to load the code from other file using PHP like
and show preview from all of those files to combines.

Due to I’m still learning about the codemirror javascript, my question is that how can I write code for EACH TEXT area to link to another file!

Another word mean how to write code to set a value of each text area to the other file!

for sample

<section id="code_editors">
    <div id="html" class="code_box">
      <textarea name="html"></textarea>  // set value to "/home/index.php"
    <div id="css" class="code_box">     
      <textarea name="css"></textarea>   // want to set value or link to 'css/main.css"
    <div id="js" class="code_box">
      <textarea name="js"></textarea>    // want to set value or link to 'shopping/main.js"

many thanks.