PHP: How to get the value of an unknown textbox?

As far as I can see, I think you are having a problem submitting the form. And as per your description and code, it looks like you are creating multiple forms.

• Not in your form method = "post"
  
  , so I added an action.
• To submit each form separately, I added an ID that is unique to each form using a spinner.
• Added a button ID that contains a spinner number, so each form has a unique form ID and a button ID, so you can now submit this form separately.
• You can now submit the form using PHP.
• I added a script to submit the form. which first grab the button id and then using that button id it will submit the form.
• And finally, check if validation is correct before submitting if you like.

Here is the code:

$counter = 0;
foreach($clients as $client) {
echo '
<div class="card">
<div class="card-header" role="tab" id="heading'. $counter .'">
    <h5 class="mb-0">
    <div class="row">
        <div class="d-flex align-items-center schedule-flex">
            <div class="col-8 col-md-10">
                <a data-toggle="collapse" data-parent="#accordion" href="#collapse'. $counter .'" aria-expanded="true" aria-controls="collapse'. $counter .'">
                    '. $client['fName'] .' '. $client['lName'] .'
                </a>
            </div>
        </div>
    </div>
    </h5>
</div>
<div id="collapse'. $counter .'" class="collapse" role="tabpanel" aria-labelledby="heading'. $counter .'">
<div class="card-block">
    <form action="editClient.php" method="post" id="form'. $counter .'">
        <div class="row">
            <div class="col-12 col-md-6">
                <label for="fname">First Name</label>
                <input name="fname" class="form-control" type="text" id="fname'. $counter .'" value="'. $client['fName'] .'" disabled>
            </div>
            <div class="col-12 col-md-6">
                <label for="lname">Last Name</label>
                <input name="lname" class="form-control" type="text" id="lname'. $counter .'" value="'. $client['lName'] .'" disabled>
            </div>
            <div class="col-12 col-md-6">
                <label for="address">Address</label>
                <input name="address" class="form-control" type="text" id="address'. $counter .'"  value="'. $client['address'] .'" disabled>
            </div>
            <div class="col-12 col-md-6">
                <label for="phone">Phone Number</label>
                <input name="phone" class="form-control" type="text" id="phone'. $counter .'" value="'. $client['telephone'] .'" disabled>
            </div>
            <div class="col-12 col-md-6">
                <label for="email">Email Address</label>
                <input name="email" class="form-control" type="text" id="email'. $counter .'" value="'. $client['email'] .'" disabled>
            </div>
        </div>
        <button class="btn btn-lg btn-primary btn-block schedule-submit edit_client_button" id="'. $counter .'" type="submit">Edit Client</button>
        <br />
    </form>
</div>
</div>
</div>
';
$counter++;


// Saperatedly add the Script to with your javascript files. I have added here just for example. put it in a right place.

<script type="text/javascript">    

    $(".edit_client_button").click(function() {
        var buttonid = this.id;             
        $( "#form"+buttonid ).submit();     
    });

</script>

      

        
        
        
      

    

I have added an attribute id

to each input you want to get the value from on click. Updated script to get input value on button click.

<script type="text/javascript">    

    $(".edit_client_button").click(function() {
        var buttonid = this.id;             
        var fname = $('#fname'+buttonid); //Kevin
        var lname = $('#lname'+buttonid); //Smith
        var address = $('#address'+buttonid); //1212 Test Street
        var phone = $('#phone'+buttonid); //8472934569 
        var email = $('#email'+buttonid); //kevin.smith@gmail.com



        // This variabls contail the input value which you want on click. 

        // Use console.log(); to see the value or simply alert();
        // example: console.log(email); or alert(email);

    });

</script>

      

        
        
        
      

    

As per your comment, I didn't get you right. You mentioned that you need to enter a value form. Example:

If I click the "change client" button inside Kevin Smith, I would for example for my php script to grab the values ​​inside the textbox, i.e. Kevin, Smith, 1212 Test Street, 2929292929 and test.test@gmail.com.

My suggested script gives you all the value in the form, however, if you want to get this input value separately, I have an updated answer code, please check this.