set_rules()

public function submit(): void {
    $this->trongate_security->make_sure_allowed();
    
    $submit = post('submit', true);
    
    if ($submit === 'Submit') {
        // Define validation rules for each field
        $this->validation->set_rules('task_title', 'Task Title', 'required|min_length[2]|max_length[255]');
        $this->validation->set_rules('task_description', 'Task Description', 'required|min_length[2]');
        
        // Run all validations
        if ($this->validation->run() === true) {
            // Validation passed - process the form
            $data = [
                'task_title' => post('task_title', true),
                'task_description' => post('task_description', true)
            ];
            
            $this->db->insert($data, 'tasks');
            redirect('tasks/manage');
        } else {
            // Validation failed - redisplay form with errors
            $this->create();
        }
    }
}

public function submit_registration(): void {
    // Username validation
    $this->validation->set_rules('username', 'Username', 'required|min_length[3]|max_length[50]|callback_check_username_available');
    
    // Email validation with DNS checking
    $this->validation->set_rules('email', 'Email Address', 'required|valid_email');
    
    // Password validation with confirmation
    $this->validation->set_rules('password', 'Password', 'required|min_length[8]|max_length[255]');
    $this->validation->set_rules('password_confirm', 'Password Confirmation', 'required|matches[password]');
    
    // Age validation
    $this->validation->set_rules('age', 'Age', 'required|integer|greater_than[17]|less_than[120]');
    
    if ($this->validation->run() === true) {
        // Process registration...
    } else {
        $this->register(); // Redisplay with errors
    }
}

public function submit_profile_photo(): void {
    // Validate uploaded image
    $this->validation->set_rules('profile_photo', 'Profile Photo', 'required|allowed_types[jpg,jpeg,png]|max_size[2048]|square|max_width[1000]|max_height[1000]');
    
    if ($this->validation->run() === true) {
        // Process file upload...
        $file = $_FILES['profile_photo'];
        // Handle file storage...
    } else {
        $this->upload_form();
    }
}

public function submit(): void {
    // Use custom validation callback
    $this->validation->set_rules('username', 'Username', 'required|callback_check_username_unique');
    
    if ($this->validation->run() === true) {
        // Username is unique and valid...
    }
}

/**
 * Custom validation callback to check username uniqueness
 * 
 * @param string $submitted_username The username to validate
 * @return string|bool Returns error message string if invalid, true if valid
 */
public function check_username_unique(string $submitted_username): string|bool {
    $existing = $this->db->get_one_where('username', $submitted_username, 'users');
    
    if ($existing) {
        return 'The {label} is already taken. Please choose another.';
    }
    
    return true;
}

public function submit_booking(): void {
    $this->validation->set_rules('start_date', 'Start Date', 'required|valid_date');
    $this->validation->set_rules('end_date', 'End Date', 'required|valid_date|callback_check_date_range');
    $this->validation->set_rules('check_in_time', 'Check-in Time', 'required|valid_time');
    
    if ($this->validation->run() === true) {
        // Process booking...
    }
}

public function check_date_range(string $end_date): string|bool {
    $start = post('start_date', true);
    $end = post('end_date', true);
    
    if (strtotime($end) <= strtotime($start)) {
        return 'The {label} must be after the Start Date.';
    }
    
    return true;
}

// All rules must pass for validation to succeed
$this->validation->set_rules('email', 'Email Address', 'required|valid_email|max_length[254]');

// If 'required' fails, other rules are still checked
$this->validation->set_rules('phone', 'Phone Number', 'required|exact_length[10]|numeric');

// Combining length and comparison rules
$this->validation->set_rules('price', 'Price', 'required|numeric|greater_than[0]|less_than[10000]');

<!-- In your view -->
<?php
echo form_open_upload('documents/submit');
echo form_label('Document');
echo form_file_select('document');
echo form_submit('submit', 'Upload');
echo form_close();
?>

// In your controller
public function submit(): void {
    $this->validation->set_rules('document', 'Document', 'required|allowed_types[pdf,doc,docx]|max_size[5120]');
    
    if ($this->validation->run() === true) {
        // File passed validation - safe to process
        $file = $_FILES['document'];
        $destination = APPPATH . 'public/uploads/' . $file['name'];
        move_uploaded_file($file['tmp_name'], $destination);
        
        set_flashdata('Document uploaded successfully');
        redirect('documents/manage');
    } else {
        $this->upload_form();
    }
}

<!-- Display all errors at top of form -->
<?= validation_errors() ?>

<!-- Or display inline per-field errors -->
<?php
echo form_label('Email');
echo validation_errors('email');
echo form_input('email', post('email', true));
?>