How to compare digits with date PHP - php

Question

Is there a way to convert values to date? I have 3 dropdown lists: day, month and year. If the selected date < today, then the consumers won't be able to click on the buy button til the day i selected in de dropdown.
The problem is, that the date I selected, is a value and not a date. Is there a way to compare my value with the date of today? The code below is the backend of my website, the if statement has to stay on the frontend
$releasedatumdag= get_post_meta( $domeinnaam_extensies->ID, 'releasedatumdag', true );
$releasedatummaand= get_post_meta( $domeinnaam_extensies->ID, 'releasedatummaand', true );
$releasedatumjaar= get_post_meta( $domeinnaam_extensies->ID, 'releasedatumjaar', true );
<tr>
<th>Releasedatum:</th>
<td>
<select name="domeinnaam_extensies_releasedatumdag">
<?php
for($idag = 1; $idag <= 31; $idag++){
echo '<option value="'.$idag.'">'.$idag.'</option>';
}
?>
</select>
<select name="domeinnaam_extensies_releasedatummaand">
<?php
for($imaand = 1; $imaand <= 12; $imaand++){
echo '<option value="'.$imaand.'">'.$imaand.'</option>';
}
?>
</select>
<select name="domeinnaam_extensies_releasedatumjaar">
<?php
for($ijaar = 2000; $ijaar < date("Y")+10; $ijaar++){
echo '<option value="'.$ijaar.'">'.$ijaar.'</option>';
}
?>
</select>
</td>
</tr>

Answer 1

You can use DateTime() to create your date. You then can compare it to another DateTime object representing today since DateTime objects are comparable:
$my_date = DateTime::createFromFormat('d/m/Y', '31/12/2014');
$now = new DateTime();
if ($my_date < $now) {
// do something
}

Answer 2

You have a day, month, and a year so just create a DateTime object and you can get the timestamp for it and compare it to today.
http://www.php.net/manual/en/datetime.construct.php
http://php.net/manual/en/function.checkdate.php
Can string the digits into any format that you want
$dateString = $releasedatumdag . '/' . $releasedatummaand . '/' . $releasedatumjaar;
//Make sure that they didn't try to create Feb. 31
if(checkdate($dateString)) {
$submittedTime = new DateTime($dateString);
$today = new DateTime();
//DO YOUR COMPARISON HERE
}