How to decode JSON in PHP and insert it into Mysql table?

I'm using the following code in PHP.

Its creating the file post.json, but inserting values into the table is not reflecting.

<?php
$input = file_get_contents('php://input');
logToFile("post.json",$input);

$json_a = json_decode($input,true);

$con = mysql_connect("localhost","root","root");
if (!$con)
{
    die('Could not connect: ' . mysql_error());
}

mysql_select_db("mydb", $con);

mysql_query("INSERT INTO order_table (Table_id, Menu_Item_Name, Menu_Item_Price, Menu_Item_quantity) VALUES('$json_a[TableId]','$json_a[ItemName]','$json_a[ItemPrice]','$json_a[ItemQuantity]')");


function logToFile($filename,$msg)
{
    $fd = fopen($filename,"a");
    $str="[".date("Y/m/d h:i:s")."]".$msg;
    fwrite($fd,开发者_JS百科$str."\n");
    fclose($fd);
}

?>

Try to replace '$json_a[foo]' with '" . $json_a['foo'] . "' or '${json_a['foo']}'.