How to Fix a PHP AJAX JQUERY Return PHP Error

When I first started working with AJAX in jQuery, one of the most frustrating problems I faced was returning errors from PHP and showing them properly on the front-end. I expected the success() callback to handle everything, but sometimes it never fired at all. After digging deeper, I realized the issue was not with jQuery it was with what PHP was returning.

Let me walk you through what went wrong in my project, how I fixed it, and what I learned along the way more: marketing internetowy.

The Problem in My First Code

Here’s the first code I wrote:

JavaScript

$.ajax({ 
    type: "POST", 
    url: "php/verifyName.php", 
    data: { name: name },
    dataType: "json",       
    success: function(data, status, xhr) {                      
        if (xhr.getResponseHeader("NB_SUCCESS") == 0) {
            valid = true;
        } else if (xhr.getResponseHeader("NB_SUCCESS") == 2) {
            alert("Erreur dans la verification !");
            valid = false;
        } else if (xhr.getResponseHeader("NB_SUCCESS") == 1) {
            alert("Le nom choisi existe déjà, veuillez choisir un autre nom !");
            valid = false;
        }   
    },
    error: function(xhr) {
        alert ("Oops: " + xhr.statusText);
    }
});

PHP

header('Content-type: application/json');

if ($nb != 0) {
    header("NB_SUCCESS: 1"); 
} else {
    header("NB_SUCCESS: 0"); 
}

Why This Fail

• I told jQuery: dataType: "json".
• But PHP only sent headers, not a JSON body.
• jQuery tried to parse the empty response as JSON → failed → threw a parsererror.
• The code skipped success() and went straight to error().

At that point, I realized: if I promise JSON, I must actually return JSON.

The Fix Always Return JSON

Here’s how I fixed it by making PHP output valid JSON.

PHP (verifyName.php)

<?php
header('Content-Type: application/json; charset=utf-8');

$name = $_POST['name'] ?? '';
$nb   = rand(0, 1); // Example: simulate database result

if ($nb != 0) {
    echo json_encode([
        'ok'      => false,
        'code'    => 'NAME_EXISTS',
        'message' => 'Le nom choisi existe déjà, veuillez choisir un autre nom.',
        'nb'      => $nb
    ]);
} else {
    echo json_encode([
        'ok'   => true,
        'code' => 'OK',
        'message' => 'Nom disponible ',
        'nb'   => $nb
    ]);
}
exit;

JavaScript

$.ajax({
  type: "POST",
  url: "php/verifyName.php",
  data: { name },
  dataType: "json"
})
.done(function (data, status, xhr) {
  if (!data.ok) {
    alert(data.message || "Erreur dans la vérification !");
    $("#result").text(" " + data.message).css("color", "red");
  } else {
    $("#result").text(" " + data.message).css("color", "green");
  }
})
.fail(function (xhr, textStatus, errorThrown) {
  console.error('AJAX failed:', textStatus, errorThrown);
  console.error('Response:', xhr.responseText);
  alert("Oops: " + (xhr.statusText || textStatus));
})
.always(function () {
  console.log("AJAX request completed.");
});

This time, the success() (or .done()) callback worked exactly as I wanted.

Making It More Practical

Once I got the basics working, I added more functionality to make the project feel professional.

Dynamic Result Display

Instead of spamming alerts, I displayed the results inline:

<input type="text" id="username" placeholder="Enter username">
<button id="checkBtn">Check</button>
<div id="result"></div>

Client-side Validation

Before sending the AJAX request, I validated the input:

$("#checkBtn").on("click", function() {
  let name = $("#username").val().trim();
  if (!name) {
    $("#result").text(" Please enter a name").css("color", "orange");
    return;
  }
  // run the AJAX here
});

Better Error Handling

I used HTTP error codes in PHP to signal real failures:

if ($db_connection_failed) {
    http_response_code(500);
    echo json_encode(['ok' => false, 'message' => 'Database connection error']);
    exit;
}

Multiple Error Codes

I returned different error codes for different issues:

echo json_encode([
    'ok' => false,
    'code' => 'INVALID_INPUT',
    'message' => 'Nom invalide. Seuls les caractères alphanumériques sont autorisés.'
]);

Final Thought

I learned that most AJAX “errors” are not really JavaScript problems they come from how the server responds. Once I understood that jQuery expects proper JSON when I ask for dataType: "json", everything became easier.

Now, instead of wasting time chasing parser errors, I structure my PHP responses properly, add error codes, and give users helpful feedback. If you’re stuck with error() firing instead of success(), double-check what your server is really sending you might be surprised.