What is the correct way to display a value in a link using php?
The (Advanced Custom Fields) php works correctly outside of the html.
Book on Expedia.com <a class="alignright" href="<?php the_field( 'expedia' ); ?>" target="_blank" ><input class="resume_contact_button" type="button" value="<?php the_field( 'low_rate' ); ?>"></a>
If the function returns a printed value:
<a href="<?php bloginfo('url'); ?>">Link</a>
If not (using the echo statement):
<a href="<?php echo get_stylesheet_directory_uri(); ?>">Link</a>
You can also use print() as part of a more complex expression where echo cannot.
By the contrary, when you want to use HTML into PHP, I recommend to close PHP and open it again after the HTML code, instead of using echo "<a href='#'></a>"; like this example:
<?php my_function() { ?>
<a href="<?php bloginfo('url'); ?>">Link</a>
<?php } ?>
Your PHP code is ok, but probably the function returns an empty value. Also, close the INPUT tag with />.
echoafterphptab like<?php echo the_field('...'); ?>the_...functions in ACF output the field already.<?php echo field( 'low_rate' ); ?>