Error inserting records in two different tables from the same model

Description of the problem

The error message you are receiving is because you are not correctly handling errors that may occur during SQL queries.

  

Call to a member function bind_param() on boolean

It is telling you that in the line $stmt2->bind_param() the variable $stmt2 contains a Boolean data and not a mysqli resource with a SQL query.

That's because the call to $db->prepare() failed:

  

Return values: mysqli_prepare() returns a statement object or FALSE if an error occurs.

Managing errors

To see the error messages that occur, you must change the code as follows:

$stmt2 = $db->prepare("
  INSERT INTO puntuacion (
    id_actividad,
    id_usuario_docente,
    id_usuario_evaluador,
    puntuacion_obtenida,
    puntuacion_evaluada,
    observacion,
    fecha
  ) VALUES (
    ?,
    ?,
    ?,
    ?,
    ?,
    ?,
    now()
  )
");
/* Si falló la consulta mostramos el mensaje de error */
if ($stmt2 === false) {
  die($db->error);
}

The same must be done after the $stmt2->execute() (it also returns false in case of error):

if ($stmt2->execute() === false) {
  die($stmt2->error);
}

You must do this with $stmtp as well as $stmtp2 .

Root of the problem

Also, when you are going to perform nested queries you must download the buffer from the previous query by $stmt->store_result() :

/* Primero ejecutamos la consulta (verificando si falló) */
$stmt->execute();
/* Después almacenamos el resultado */
$stmt->store_result();
/* Por último asociamos los campos con variables para el fetch() */
$stmt->bind_result($id_actividad);
/* Ahora podemos trabajar con los resultados */
while ($stmt->fetch()) { ... }

The order is important and there is a note in the $mysqli::bind_result() documentation that states it this way:

  

Note: Note that all columns must be linked after calling mysqli_stmt_execute() and before calling mysqli_stmt_fetch() .

Remaining the code as follows:

<?php
error_reporting(E_ALL);
require_once 'conexion.php';
$obj = json_decode(file_get_contents('php://input'));

$stmt = $db->prepare("
  INSERT INTO usuario (
    id_rol,
    nombre,
    apellidoP,
    apellidoM,
    correo,
    pass,
    campus
  ) VALUES (
    ?,
    ?,
    ?,
    ?,
    ?,
    ?,
    ?
  )"
);
/* Si falló la consulta mostramos el mensaje de error */
if ($stmt === false) {
    die($db->error);
}

$stmt->bind_param('issssss',
  $obj->id_rol,
  $obj->nombre,
  $obj->apellidoP,
  $obj->apellidoM,
  $obj->correo,
  $obj->pass,
  $obj->campus
);

if ($stmt->execute() === false) {
    die($stmt->error);
}
/* Obtenemos el id de la inserción */
$idGenerado = $stmt->insert_id;
/* Ya no necesitamos de nuevo la consulta preparada */
$stmt->close();

if($obj->id_rol == 3) {
    $stmt = $db->prepare("SELECT id_actividad FROM actividad");
    /* Si falló la consulta mostramos el mensaje de error */
    if ($stmt === false) {
        die($db->error);
    }
    if ($stmt->execute() === false) {
        die($stmt->error);
    }
    /* Descargamos los datos para poder hacer consultas anidadas */
    $stmt->store_result();
    /* Asignamos a $id_actividad el campo por cada fetch() */
    $stmt->bind_result($id_actividad);
    /* Iteramos los resultados */
    while ($stmt->fetch()) {
        echo $id_actividad, " ", $idGenerado, PHP_EOL;
        $evaluador = 27;
        $puntuacion1 = 0;
        $puntuacion2 = 0;
        $observacion = "0";
        $stmt2 = $db->prepare("
          INSERT INTO puntuacion (
            id_actividad,
            id_usuario_docente,
            id_usuario_evaluador,
            puntuacion_obtenida,
            puntuacion_evaluada,
            observacion,
            fecha
          ) VALUES (
            ?,
            ?,
            ?,
            ?,
            ?,
            ?,
            now()
          )
        ");
        /* Si falló la consulta mostramos el mensaje de error */
        if ($stmt2 === false) {
            die($db->error);
        }
        $stmt2->bind_param('siiiis',
          $id_actividad,
          $idGenerado,
          $evaluador,
          $puntuacion1,
          $puntuacion2,
          $observacion
        );
        if ($stmt2->execute() === false) {
            die($stmt2->error);
        }
        $stmt2->close();
    }
    $stmt->close();
}
echo "registro almacenado";