Código:
<select title="Es necesario añadir una tarea" required="required" name="tarea" id="tarea">
<?php
$list_tareas = "SELECT p1.* , p2.* FROM tareas AS p1 , proyectos AS p2 ";
$select_tareas = mysqli_query($conexion, $list_tareas);
while ($tareas = mysqli_fetch_array($select_tareas)){
echo "<option value = '" . $tareas['id_tarea'] . "'>Tarea " . $tareas['nombre_tarea'] . " del proyecto ". $tareas['nombre_proyecto'] ."</option>";
}
?>


