Checkbox, Radio and Select Inputs

Checkboxes, radio buttons, and select lists let users choose from options instead of typing. They look simple, but each sends data differently — or not at all when nothing is chosen — and the submitted values can be manipulated just like text fields. This lesson shows how each input submits data, how to validate choices safely, and how to re-select the user's choices when a form is shown again.

How Each Input Submits Data

Key points:

• The value attribute is what PHP receives — not the visible label.
• Unchecked checkboxes, unselected radio groups, and empty multi-selects send nothing: the key is missing from $_POST.
• Names ending in [] produce arrays.

Single Checkbox

Reading a Checkbox

<?php
// Request body: newsletter=1   (checkbox checked)
// or:           (empty)        (checkbox unchecked)

$wantsNewsletter = ($_POST['newsletter'] ?? null) === '1';

Because an unchecked checkbox is not sent, ?? null provides a default, and the strict comparison checks for the exact expected value.

The Hidden Field Technique

Sometimes you want the field to be present either way, for example when saving settings:

<input type="hidden" name="newsletter" value="0">
<input type="checkbox" name="newsletter" value="1">

• Unchecked: only the hidden field is sent → '0'.
• Checked: both are sent with the same name; PHP keeps the last value → '1'.

This makes "the user unchecked it" ('0') distinguishable from "the field was not part of this form" (missing).

Checkbox Groups

<label><input type="checkbox" name="courses[]" value="php"> PHP Basics</label>
<label><input type="checkbox" name="courses[]" value="mysql"> MySQL</label>
<label><input type="checkbox" name="courses[]" value="laravel"> Laravel</label>

If the user checks PHP and Laravel, PHP receives:

$_POST['courses'] = ['php', 'laravel'];

Validation must check that:

1. The value is an array (a manipulated request could send a string).
2. Every item is one of the allowed values.
3. The number of selections is within any limits (at least one, at most three, …).

Radio Buttons

<label><input type="radio" name="batch" value="morning"> Morning (9–12)</label>
<label><input type="radio" name="batch" value="evening"> Evening (6–9)</label>

All radio buttons in a group share the same name, so at most one value is sent. If none is selected, the key is missing. Validation is a single allow-list check:

$batch = $_POST['batch'] ?? null;
$isValid = in_array($batch, ['morning', 'evening'], true);

Select Lists

<select name="city">
    <option value="">-- Choose a city --</option>
    <option value="MAA">Chennai</option>
    <option value="CJB">Coimbatore</option>
    <option value="IXM">Madurai</option>
</select>

• A select always sends the value of its selected option. The placeholder option uses an empty value, so "no real choice" arrives as ''.
• <optgroup label="Tamil Nadu"> groups options visually; it does not change what is submitted.
• With multiple and a name ending in [], several values are sent as an array.

Defining Options in PHP

Define the options once in an array whose keys are the submitted values and whose values are the labels. The same array is then used to render the form and to validate the input:

$cities = ['MAA' => 'Chennai', 'CJB' => 'Coimbatore', 'IXM' => 'Madurai'];

$isValidCity = array_key_exists($_POST['city'] ?? '', $cities);

This guarantees that the form and the validation never disagree about which choices exist.

Practical Example: Course Registration

Validation

<?php
const COURSES = ['php' => 'PHP Basics', 'mysql' => 'MySQL', 'laravel' => 'Laravel'];
const BATCHES = ['morning' => 'Morning (9–12)', 'evening' => 'Evening (6–9)'];
const CITIES = ['MAA' => 'Chennai', 'CJB' => 'Coimbatore', 'IXM' => 'Madurai'];

function validateRegistration(array $input): array
{
    $errors = [];

    $courses = $input['courses'] ?? [];
    $courses = is_array($courses) ? array_values(array_unique(array_filter($courses, 'is_string'))) : [];

    if ($courses === []) {
        $errors['courses'] = 'Select at least one course.';
    } elseif (array_diff($courses, array_keys(COURSES)) !== []) {
        $errors['courses'] = 'Select only courses from the list.';
    }

    $batch = $input['batch'] ?? '';
    if (!is_string($batch) || !array_key_exists($batch, BATCHES)) {
        $errors['batch'] = 'Choose a batch.';
    }

    $city = $input['city'] ?? '';
    if (!is_string($city) || !array_key_exists($city, CITIES)) {
        $errors['city'] = 'Choose a city.';
    }

    $data = [
        'courses' => $courses,
        'batch' => is_string($batch) ? $batch : '',
        'city' => is_string($city) ? $city : '',
        'newsletter' => ($input['newsletter'] ?? '0') === '1',
    ];

    return [$data, $errors];
}

[$data, $errors] = validateRegistration([
    'courses' => ['php', 'python'],
    'batch' => 'night',
    'city' => 'IXM',
    'newsletter' => '1',
]);
print_r($errors);

[$data, $errors] = validateRegistration([
    'courses' => ['php', 'mysql', 'php'],
    'batch' => 'evening',
    'city' => 'MAA',
    'newsletter' => '0',
]);
var_dump($errors === []);
echo implode(', ', $data['courses']) . ' | ' . $data['batch'] . ' | ' . $data['city'] . ' | newsletter: ' . var_export($data['newsletter'], true) . "\n";

Output

Array
(
    [courses] => Select only courses from the list.
    [batch] => Choose a batch.
)
bool(true)
php, mysql | evening | MAA | newsletter: false

Step-by-Step Explanation

1. Courses: non-array input is treated as empty, non-string items are dropped, duplicates are removed, and every remaining value must exist in COURSES. array_diff() returns the submitted values that are not allowed; any result means a manipulated or outdated value.
2. Batch and city: each must be a string that exists as a key in its options array. The unknown batch night is rejected.
3. Newsletter: the hidden-field technique sends '0' or '1'; only '1' counts as opting in.
4. The function returns cleaned data (with duplicates removed) and field-keyed error messages.

Rendering With the User's Previous Choices

When the form is shown again after an error, the user's choices must stay selected. Small helpers keep the template readable:

<?php
function e(string $value): string
{
    return htmlspecialchars($value, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
}

function checked(bool $condition): string
{
    return $condition ? ' checked' : '';
}

function selected(bool $condition): string
{
    return $condition ? ' selected' : '';
}

$old = ['courses' => ['php', 'mysql'], 'batch' => 'evening', 'city' => 'MAA'];

const COURSES = ['php' => 'PHP Basics', 'mysql' => 'MySQL', 'laravel' => 'Laravel'];
const BATCHES = ['morning' => 'Morning (9–12)', 'evening' => 'Evening (6–9)'];
const CITIES = ['MAA' => 'Chennai', 'CJB' => 'Coimbatore', 'IXM' => 'Madurai'];

foreach (COURSES as $value => $label) {
    printf("<label><input type=\"checkbox\" name=\"courses[]\" value=\"%s\"%s> %s</label>\n",
        e($value), checked(in_array($value, $old['courses'], true)), e($label));
}

foreach (BATCHES as $value => $label) {
    printf("<label><input type=\"radio\" name=\"batch\" value=\"%s\"%s> %s</label>\n",
        e($value), checked($value === $old['batch']), e($label));
}

echo "<select name=\"city\">\n";
echo "<option value=\"\">-- Choose a city --</option>\n";
foreach (CITIES as $value => $label) {
    printf("<option value=\"%s\"%s>%s</option>\n", e($value), selected($value === $old['city']), e($label));
}
echo "</select>\n";

Output (HTML)

<label><input type="checkbox" name="courses[]" value="php" checked> PHP Basics</label>
<label><input type="checkbox" name="courses[]" value="mysql" checked> MySQL</label>
<label><input type="checkbox" name="courses[]" value="laravel"> Laravel</label>
<label><input type="radio" name="batch" value="morning"> Morning (9–12)</label>
<label><input type="radio" name="batch" value="evening" checked> Evening (6–9)</label>
<select name="city">
<option value="">-- Choose a city --</option>
<option value="MAA" selected>Chennai</option>
<option value="CJB">Coimbatore</option>
<option value="IXM">Madurai</option>
</select>

How It Works

• Each control is generated from the same option arrays used for validation.
• checked and selected are added only where the previous value matches, using strict comparison so that, for example, the value '0' is not confused with an empty selection.
• Values and labels are escaped, even though they come from your own code — a consistent habit prevents mistakes when options later come from a database.

Keeping previous input in forms is covered further in Sticky Forms.

Common Mistakes

Reading Unchecked Fields Without Defaults

Effect: "Undefined array key" warnings.

Fix: $_POST['field'] ?? null (or ?? [] for groups).

Forgetting [] in Group Names

Effect: Only the last checked value is received.

Fix: Name checkbox groups and multi-selects name="field[]".

Trusting Submitted Choices

Effect: Users submit values that were never offered (other prices, roles, or IDs).

Fix: Validate every choice against the options array.

Comparing With the Label Instead of the Value

Effect: Validation fails, or labels change break the logic.

Fix: Use stable value codes; labels are for display only.

Loose Comparisons When Re-Selecting

Effect: Wrong options appear selected (for example, 0 == '' in older code).

Fix: Use === and in_array(..., true).

Best Practices

• Define options once (value ⇒ label) and use them for both rendering and validation.
• Use [] names for multi-value inputs, and check that the received value is an array.
• Validate every submitted choice with allow-lists.
• Use the hidden-field technique when an unchecked box must be stored explicitly.
• Re-check and re-select the user's choices when redisplaying the form.

Official reference: https://www.php.net/manual/en/faq.html.php