How to check in PHP if a checkbox is checked or not before posting/submitting?

I’m trying to check if a checkbox has been selected so that if it is, I can then display another textbox, however I’ve tried the below code to try and check if the checkbox is checked or not but keep getting errors. I need to check before the form is submitted/posted as the text field that shows after needs submitting with the form.

HTML + PHP:

<div class="form-check">
    <input type="checkbox" name = "job_mileage_check" class="form-check-input" id="job_mileage">
    <label class="form-check-label" for="job_mileage">Mileage?</label>
</div>
<?php
    $check_value = isset(['job_mileage_check']) ? 1 : 0;
?>

All/Any help is appreciated.

Answer

you can try this in front but with checkbox

Form

<div id='radios'>
 <div class="form-check">
  <input type="radio" id="inputRadio1" value="data1">
  <label class="form-check-label">myText1</label>
 </div>
 <div class="form-check">
  <input type="radio" id="inputRadio2" value="data2">
  <label class="form-check-label">myText2</label>
 </div>

JS

const inputRadio1 = document.getElementById('radioinputRadio1');
const inputRadio2 = document.getElementById('radioinputRadio2');
const btnSubmit = document.getElementById('btnSubmit');

btnSubmit.addEventListener('click', e => {
 if (!inputRadio1.checked && !inputRadio2.checked) {
  // error 
  // ...
  e.preventDefault();  
 } 
});