your css page
<code lang="CSS">
.error{
	font-weight:bold;
	color:#FF0000;
}
</code>

class.error.php
<code lang="PHP">
<?php 

class error{
	
	function error(){
		$errors = array();
	}
	
	function setError($new){
		global $errors;
		$errors[] = $new;
	}
	
	function showErrors(){
		global $errors;
		if($this->countErrors() > 0){
			foreach ($errors as $err){
				echo "<div class=\"error\">$err</div>";
			}
		}
	}
	
	function countErrors(){
		global $errors;
		return count($errors);
	}
}

?>
</code>

test_error.php
<code lang="PHP">
<?php 

	require_once "class.error.php";
	$e = new error;
	
	if($_POST){
		$name = $_POST['name'];
		
		if(strlen($name) == 0){
			$e->setError("Name is not valid");
		}
		
		if($e->countErrors() == 0){
			//Since there aren't any errors process the form
			header("location: success.php");
			die();
		}
		
	}

?>
<div align="center"><?php $e->showErrors(); ?></div>
<form action="<?php echo $_SERVER['PHP_SELF'] ?>" method="post">
<table border="0" cellspacing="0" style="margin:0px auto">
    <tr>
    	<td colspan="2">Change your name:</td>
    </tr>
    <tr>
        <td>Name:</td>
        <td><input name="name" type="text" value="<?php echo $name ?>" /></td>
    </tr>
    <tr>
    	<td colspan="2" align="center"><input name="Submit" type="submit" value="Edit" /></td>
    </tr>
</table>
</form>
</code>