To add product attribute programatically, create InstallData.php and and define the method intsall() as follows

File : app/code/PHPCodez/First/Setup/InstallData.php

<?php
 namespace PHPCodez\First\Setup;
 use Magento\Eav\Setup\EavSetup;
 use Magento\Eav\Setup\EavSetupFactory;
 use Magento\Framework\Setup\InstallDataInterface;
 use Magento\Framework\Setup\ModuleContextInterface;
 use Magento\Framework\Setup\ModuleDataSetupInterface;

class InstallData implements InstallDataInterface
 {
 private $eavSetupFactory;

public function __construct(EavSetupFactory $eavSetupFactory) {
 $this->eavSetupFactory = $eavSetupFactory;
 }
 
 public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
 {
 $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
 $eavSetup->addAttribute(
 \Magento\Catalog\Model\Product::ENTITY,
 'phpcodez_ratig',
 [
 'type' => 'text',
 'backend' => '',
 'frontend' => '',
 'label' => 'PHPCodez Rating',
 'input' => 'text',
 'class' => '',
 'source' => '',
 'global' => \Magento\Eav\Model\Entity\Attribute\ScopedAttributeInterface::SCOPE_GLOBAL,
 'visible' => true,
 'required' => true,
 'user_defined' => false,
 'default' => '',
 'searchable' => false,
 'filterable' => false,
 'comparable' => false,
 'visible_on_front' => false,
 'used_in_product_listing' => true,
 'unique' => false,
 'apply_to' => ''
 ]
 );
 }
 }

Purge cache and deploy static content by running the following command.

php bin/magento setup:upgrade
php bin/magento setup:static-content:deploy -f

Now go to admin panel and you can see the new attribute added in the product add/edit section.