validateform.php

<?php
//defining null values to variables and empty sets
$name=$email=$age="";
$nameErr=$emailErr=$ageErr="";

if($_SERVER['REQUEST_METHOD']=="POST"){
	if (empty($_POST['name'])){
		$nameErr="name is required";
	}
	else{
		$name=test_input($_POST['name']);
		if(!preg_match("/^[a-zA-Z]*$/", $name)){
			$nameErr="only letters and spaces allowed";
		}
	}
	//email validation
	if(empty($_POST['email'])){
		$emailErr="email is required";
	}
	else{
		$email=test_input($_POST['email']);
		if (!filter_var($email,FILTER_VALIDATE_EMAIL)){
			$emailErr="invalid email format";
		}
	}
	//age validation
	if(empty($_POST['age'])){
		$ageErr="age is required";
	}
	else{
		$age=test_input($_POST['age']);
		if(!is_numeric($age)|| $age<1 || $age>120){
			$ageErr="enter a valid age between 1 and 120";
		}
	}
}


//function to clean user input
function test_input($data){
	$data=trim($data);
	$data=stripslashes($data);
	$data=htmlspecialchars($data);
	return $data;
}

?>

<form method="POST" action="<?php echo htmlspecialchars($_SERVER['PHP_SELF']);?>">
	name:<input type="text" name="name" value="<?php echo $name;?>">
	<span style="color:red;">* <?php echo $nameErr;?></span><br><br>

	email:<input type="text" name="email" value="<?php echo $email;?>">
	<span style="color:red;">* <?php echo $emailErr;?></span><br><br>

	age:<input type="number" name="age" value="<?php echo $age; ?>">
	<span style="color:red">* <?php echo $ageErr; ?></span><br><br>

	<input type="submit" value="submit">
</form>