Change basic php to if statement php

the_sub_field() function may produce output (the link), or not.

The first thing to do is capture that output using an output buffer in order to check it's contents:

<?php
    ob_start();
    the_sub_field( 'funding_link' );
    $link = ob_get_clean();

Next just check the content of $link: if not an empty string then output the button's HTML code

    if( $link != "" )
    {
        echo "<div class=\"button\"><a href=\"$link\">Donate</a></div>";
    }

Finally close the PHP code block

?>

If you're using advanced custom fields then

the_sub_field(...)

is equivalent to

echo get_sub_field(...)

In this case the solution is simplier as you can just get the link into a variable without using a buffer:

<?php
    $link = get_sub_field( 'funding_link' );
    if( $link != "" )
    {
        echo "<div class=\"button\"><a href=\"$link\">Donate</a></div>";
    }
?>

Ok guys, I appreciate everyone's help. I figured this out with the following addition

<?php if( get_sub_field('funding_link') ): ?>
<div class="button"><a href="<?php the_sub_field( 'funding_link' ); ?>">Donate</a></div>
<?php endif; ?>

As far as i can see you have html on your php file, one way to do it is like this

<?php
    $mycondition = false;
    // your condition should be something like
    // $mycondition = the_sub_field( 'funding_link' );
    // if the string is empty it's also considered a false value
?>
<?php if($mycondition):?>
    <a href="somelink.html">Donate</a>
<?php else:?>
    <!--here goes nothing-->
<?php endif;?>

Another way to do it with a shothand if

<?=($condition)?"let's show the link":"let's show nothing"?>

Try it online!