age calculation & if inside if not working / php - php

Question

I am trying to convert three separate database columns into a date (day,month,year) and calculate the age so only users over the age of 15 or 18 can purchase certain products. The code below doesnt work as it echoes '0 days, 0 months, 0 years' and still adds the product to the basket. Which means the age calculation doesnt work, and my first if statement doesnt work either.
<?php
$username = $_SESSION['solentuser'];
echo "$username's account!<br>";
$conn = new PDO("mysql:host=localhost;dbname=u;","u","p");
$productID= htmlentities($_GET['ID']);
//startdate
$result=$conn->prepare("SELECT * FROM users WHERE username=:username");
$result->bindParam(":username",$username);
$result->execute();
$row=$result->fetch();
$birthdate = $row['yearofbirth'] . $row['monthofbirth'] . $row['dayofbirth'];
$presentdate = date('Ymd');
$birthday = new DateTime($birthdate);
$currentdate = new DateTime($presentdate);
$age = $birthday->diff($currentdate);
echo $age->format('<br>%d Days %m Months %y Years<br>');
//enddate
$results=$conn->prepare("SELECT * FROM products where ID=:productID");
$results->bindParam(":productID",$productID);
$results->execute();
$row=$results->fetch();
if($row['agelimit'] <= $age){
if($row['stocklevel'] >= 1){
$result=$conn->prepare("INSERT INTO basket(productID,username,qty) values(:productID,:username,1)");
$result->bindParam(":productID",$productID);
$result->bindParam(":username",$username);
$result->execute();
$result=$conn->prepare("UPDATE products SET stocklevel=stocklevel-1 WHERE ID=:productID");
$result->bindParam(":productID",$productID);
$result->execute();
echo "You have successfully added this product to your basket!";
}
else{
echo "This product is out of stock!";
}
}
else{
echo "You are not old enough to purchase this product!";
}
//print_r($conn->errorInfo());
?>
any suggestions as to where the error is? i have read that it is possible to write an if statement inside an if statement, so why does this one not work?
thank you!

Answer 1

I'd echo out $birthdate and verify there's a valid date there. (We aren't getting back single digits, a date of 2009-05-04 getting represented as '2009', '5' and '4' such that when we concatenate them, we get 200954, or maybe extra spaces. (We're not seeing the datatypes of the three separate columns.)
We might try adding some delimiters in there, so we'd get 2009-5-4, likely we could get that converted into a DateTime, using the correct format string.
If I had separate values for month, day and year, I would use PHP mktime, and then create a DateTime object from that.
(MySQL does provide a DATE datatype that allows for a very large range of valid dates, and doesn't allow invalid dates to be stored. Storing three separate columns to represent a single date just smells like the wrong way to do it. (If I actually needed the separate month and day columns (to allow indexing for some queries), I would add those in addition to the birthdate DATE column, not in place of it, with triggers to keep the values in sync with the birthdate column.)
Also, $age is a DateInterval object. You seem to be aware that we can use the format method to extract integer number of years.
$age_yrs = $age->format('%y');
We're guessing that the database column age_limit is integer years.
if( $row['agelimit'] <= $age_yrs ) {
Right before that if statement, we can confirm that what we think to be true is actually true...
echo " age_yrs=" . $age_yrs;
echo " row_age_limit=" . $row['age_limit'];
Looking closely at the debugging output helps us identify if the problem is before the if statement or after, so we aren't chasing down a problem in a section of code where there isn't a problem, the problem is somewhere else, on a preceding line.
I encourage you to develop the skills needed to debug programs that you write. It seems like you are making some (wrong) assumptions about what the variables are containing.
Adding echo and var_dump during development is a first step in verifying that what you think to be true is actually true.
I'd go as far as recommending that you look at every line of code you write as possibly going wrong, especially in edge cases.
https://ericlippert.com/2014/03/05/how-to-debug-small-programs/
(StackOverflow is a question/answer community, not a debugging service.)