使用PHP表单提交以增加运算符添加JSON

I'm using a form to submit data and store it as JSON in a file. No I want to add a new line of info called 'age', and have PHP incrementing it as data is stored... first form submit would have 'age':0, 2nd would have 'age':1 and so on.

$age = 0;

$formdata = array(
    'job'=> $_POST['job'],
    'company'=> $_POST['company'],
    'city'=> $_POST['city'],
    'category'=> $_POST['category'],
    'type'=> $_POST['type'],
    'description'=> $_POST['description'],
    'age'=> $age++
);

I have this above, but it keeps storing data as 'age':0, no incrementing. How should I fix it?