Passing data via Modal Bootstrap and getting php variable?

<button type='button' data-a="<?echo $fila['idPlaza'];?>" href='#editarUsuario' class='modalEditarUsuario btn btn-warning btn-xs' data-toggle='modal' data-backdrop='static' data-keyboard='false' title='Editar usuario'>
    <img src='../images/edit.png' width='20px' height='20px'>
</button>

Put below code in footer before end of </body> tag.

<!-- MODAL EDITAR-->
<div id="editarUsuario" class="modal fade modal" role="dialog">
    <div class="vertical-alignment-helper">
        <div class="modal-dialog vertical-align-center">
        <div class="modal-content">


        </div>
        </div>
    </div>
</div>

Use Script like this.

<script>
    $('.modalEditarUsuario').click(function(){
        var ID=$(this).attr('data-a');
        $.ajax({url:"NewPage.php?ID="+ID,cache:false,success:function(result){
            $(".modal-content").html(result);
        }});
    });
</script>

Create NewPage.php, and paste the below code. (Remember, this page name NewPage.php is used in script tag also. if you are planning to change the name of this page, change page name there in script tag too. Both are related.)

<?
$ID=$_GET['ID'];
?>
<div class="modal-header">
   <button type="button" class="close" data-dismiss="modal">&times;</button>
   <h2 class="modal-title">Editar usuario</h2>
</div>
<form name="formularioModificaUsuario" method='post' action="usuarios.php">
    <div class="modal-body">
        <input type="text" class="form-control" name="idPlaza" id="a">
        <?php 
            $valueA = $ID;
        ?>
    </div>
    <div class="modal-footer">
        <button type="reset" id="cancelar" class="btn btn-danger" data-dismiss="modal" value="reset">
        <img src='../images/cancel.png' width='20px' height='20px'> Cancelar</button>
        <button type="submit" name="submitModifica" id="submit" class="btn btn-primary" value="submit">
        <img src='../images/save_changes.png' width='20px' height='20px'> Guardar cambios</button>
    </div>
</form>