To add customer attribute programatically, create InstallData.php and and define the method install() 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;
 use Magento\Eav\Model\Config;
 use Magento\Customer\Model\Customer;

class InstallData implements InstallDataInterface
 {
 private $eavSetupFactory;

public function __construct(EavSetupFactory $eavSetupFactory, Config $eavConfig)
 {
 $this->eavSetupFactory = $eavSetupFactory;
 $this->eavConfig = $eavConfig;
 }

public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
 {
 $eavSetup = $this->eavSetupFactory->create(['setup' => $setup]);
 $eavSetup->addAttribute(
 \Magento\Customer\Model\Customer::ENTITY,
 'phpcodez_attribute',
 [
 'type' => 'varchar',
 'label' => 'PHPCodez Rating',
 'input' => 'text',
 'required' => false,
 'visible' => true,
 'user_defined' => true,
 'position' => 999,
 'system' => 0,
 ]
 );
 $sampleAttribute = $this->eavConfig->getAttribute(Customer::ENTITY, 'phpcodez_attribute');

// more used_in_forms ['adminhtml_checkout','adminhtml_customer','adminhtml_customer_address','customer_account_edit','customer_address_edit','customer_register_address']
 $sampleAttribute->setData(
 'used_in_forms',
 ['adminhtml_customer']

);
 $sampleAttribute->save();
 }
 }

Now purche cache and deploy static content by running the following command.

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