<?php
$format = 'YYYY-MM-DD';
$query = $db->prepare("SELECT TO_CHAR(NOW(), '" . $format . "') AS DATETIME");

if($query->execute()) {
    echo 'valid';
else
    echo 'invalid';

//Result valid;

<?php
$format = 'abcdefg';
$query = $db->prepare("SELECT TO_CHAR(NOW(), '" . $format . "') AS DATETIME");

if($query->execute()) {
    echo 'valid';
else
    echo 'invalid';
//Result valid

//sql
SELECT TO_CHAR(NOW(), 'YYYY-MM-DD') AS DATETIME;
//query executed
//Result
2018-11-16

//sql
SELECT TO_CHAR(NOW(), 'abcdefg') AS DATETIME;
//query executed
    //Result
    aad6efg