Are there any specific PHP functions or techniques that can be used to change the appearance of past dates, such as making them smaller or changing their color?

To change the appearance of past dates in PHP, you can use the date_diff function to compare the current date with the date you want to format. Based on the result of the comparison, you can apply different styles or classes to the date element in your HTML output. 

<?php
$today = new DateTime();
$someDate = new DateTime('2021-01-01');

$interval = $today->diff($someDate);
if ($interval->invert == 1) {
    echo '<span style="font-size: 10px; color: gray;">' . $someDate->format('Y-m-d') . '</span>';
} else {
    echo $someDate->format('Y-m-d');
}
?>

Keywords

PHP date formatting CSS styling manipulation

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