[Parse error: syntax error, unexpected '' Please fill all the cam '(T_CONSTANT_ENCAPSED_STRING) in C: \ xampp \ htdocs \ index.php on line 8]

I already saw the error, you have some other keys in your code, try this one.

<?php
session_start();
include_once "conexion.php";
    if($_POST['usuario'] == '' or $_POST['password'] == '' or $_POST['repassword'] == '')

    {
        echo 'Por favor llene todos los campos.';
    }
    else
    {
        $sql = 'SELECT * FROM usuarios';
        $rec = mysql_query($sql);
        $verificar_usuario = 0;

        while($result = mysql_fetch_object($rec))
        {
            if($result->usuario == $_POST['usuario'])
            {
                $verificar_usuario = 1;
            }
        }

        if($verificar_usuario)
        {
            if($_POST['password'] == $_POST['repassword'])
            {
                $usuario = $_POST['usuario'];
                $password = $_POST['password'];
                $sql = "INSERT INTO usuarios (usuario,password) VALUES ('$usuario','$password')";
                mysql_query($sql);

                echo 'Usted se ha registrado correctamente.';
            }
            else
            {
                echo 'Las claves no son iguales, intente nuevamente.';
            }
        }
        else
        {
            echo 'Este usuario ya ha sido registrado anteriormente.';
        }
    }

Try and comment on that.

The mysql_connect () driver is obsolete since PHP 5.5.0 and removed from PHP 7.0.0. You should use the mysqli_connect driver.

link