function register( $user, $pass, $pass2 )
{
	if ( isset( $_POST['register'] ) )
	{
                //Escape is a function with mysql_real_escape_string() and more
		$user = escape( $user );
		$pass = escape( $pass );
		$pass2 = escape( $pass2 );
		
		if ( empty( $user ) )
		{
			$error = "Username missing!";
		}
		else if ( empty( $pass ) )
		{
			$error = "Password missing!";
		}
		else if ( empty( $pass2 ) )
		{
			$error = "Please repeat your password!";
		}
		else if ( $pass != $pass2 )
		{
			$error = "Your passwords do not match!";
		}
		else if ( strlen( $user ) > 25 )
		{
			$error = "Username is to long, 25 characters max!";
		}
		else if ( strlen( $pass ) > 25 )
		{
			$error = "Password is to long, 25 characters max!";
		}
		else if ( strlen( $pass ) < 5 )
		{
			$error = "Password is to short, 5 characters min!";
		}
                //I use trim to check if a user has a username with only whitespace, not using regex since having a whitespace in the username should be ok
		else if ( trim( $user ) == "" )
		{
			$error = "Usernames only containing white-spaces is not allowed!";
		}
		else
		{
			$fetch = mysql_query( "SELECT `user` FROM `user` WHERE `user` = '".$user."'" ) or die ( mysql_error() );
			if ( mysql_num_rows( $fetch ) != 0 )
			{
				$error = "That username is already taken!";
			}
			else
			{
				$pass = md5( $pass );
				mysql_query( "INSERT INTO `user` ( `user`, `pass` ) VALUES ( '".$user."', '".$pass."' )" ) or die ( mysql_error() );
			}
		}
	}
}

function register( $user, $pass, $pass2 )
{
	if ( isset( $_POST['register'] ) )
	{
		$user = escape( $user );
		$pass = escape( $pass );
		$pass2 = escape( $pass2 );
		
		if ( empty( $user ) )
		{
			$error = "Username missing!";
		}
		else if ( empty( $pass ) )
		{
			$error = "Password missing!";
		}
		else if ( empty( $pass2 ) )
		{
			$error = "Please repeat your password!";
		}
		else if ( $pass != $pass2 )
		{
			$error = "Your passwords do not match!";
		}
		else if ( strlen( $user ) > 25 )
		{
			$error = "Username is to long, 25 characters max!";
		}
		else if ( strlen( $pass ) > 25 )
		{
			$error = "Password is to long, 25 characters max!";
		}
		else if ( strlen( $pass ) < 5 )
		{
			$error = "Password is to short, 5 characters min!";
		}
		else if ( trim( $user ) == "" )
		{
			$error = "Usernames only containing white-spaces is not allowed!";
		}
		else
		{
			$fetch = mysql_query( "SELECT `user` FROM `user` WHERE `user` = '".$user."'" ) or die ( mysql_error() );
			if ( mysql_num_rows( $fetch ) != 0 )
			{
				$error = "That username is already taken!";
			}
			else
			{
				$pass = md5( $pass );
				mysql_query( "INSERT INTO `user` ( `user`, `pass` ) VALUES ( '".$user."', '".$pass."' )" ) or die ( mysql_error() );
				$error = "Registration complete, continue to login via the \"Back\" link below.";
			}
		}
	}
}

Simple Register Function

PHP