PHP is used to validate data on the client instead of the server? code example

Example 1: data validation of $_POST in php

<?php

class  Input {
	static $errors = true;

	static function check($arr, $on = false) {
		if ($on === false) {
			$on = $_REQUEST;
		}
		foreach ($arr as $value) {	
			if (empty($on[$value])) {
				self::throwError('Data is missing', 900);
			}
		}
	}

	static function int($val) {
		$val = filter_var($val, FILTER_VALIDATE_INT);
		if ($val === false) {
			self::throwError('Invalid Integer', 901);
		}
		return $val;
	}

	static function str($val) {
		if (!is_string($val)) {
			self::throwError('Invalid String', 902);
		}
		$val = trim(htmlspecialchars($val));
		return $val;
	}

	static function bool($val) {
		$val = filter_var($val, FILTER_VALIDATE_BOOLEAN);
		return $val;
	}

	static function email($val) {
		$val = filter_var($val, FILTER_VALIDATE_EMAIL);
		if ($val === false) {
			self::throwError('Invalid Email', 903);
		}
		return $val;
	}

	static function url($val) {
		$val = filter_var($val, FILTER_VALIDATE_URL);
		if ($val === false) {
			self::throwError('Invalid URL', 904);
		}
		return $val;
	}

	static function throwError($error = 'Error In Processing', $errorCode = 0) {
		if (self::$errors === true) {
			throw new Exception($error, $errorCode);
		}
	}
}

Example 2: data validation of $_POST in php

<?php
	
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
	
		// the request method is fine

} else {

	exit('Invalid Request');

}