Make Input Field Output Links - php

Question

So, I added an input field to one of my post types.
What I'm trying to do is put a link (google.com, facebook.com, any link) into this field, and have it output on the front end into a button.
Below is my button. I would like it to go to the link I placed on the input field when I click the button.
<?php echo __('Purchase Now','Vierra'); ?>
Here's the code for the metabox:
<?php
add_action( 'add_meta_boxes', 'cart_meta_box_add' );
function cart_meta_box_add() {
add_meta_box( 'cart-meta-box-id', 'Put cart Here!', 'cart_meta_box_cb', 'Room', 'side', 'high' );
}
function cart_meta_box_cb() {
wp_nonce_field( basename( __FILE__ ), 'cart_meta_box_nonce' );
$value = get_post_meta(get_the_ID(), 'cart_key', true);
$html = '<label>cart: </label><input type="text" name="cart" value="'.$value.'"/>';
echo $html;
}
add_action( 'save_post', 'cart_meta_box_save' );
function cart_meta_box_save( $post_id ){
if( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) return;
if ( !isset( $_POST['cart_meta_box_nonce'] ) || !wp_verify_nonce( $_POST['cart_meta_box_nonce'], basename( __FILE__ ) ) ) return;
if( !current_user_can( 'edit_post' ) ) return;
if( isset( $_POST['cart'] ) )
update_post_meta( $post_id, 'cart_key', esc_attr( $_POST['cart'], $allowed ) );
}
?>
Thank you for the help!

Answer 1

It's the same function used in the admin: get_post_meta(). Assuming the button is being printed inside the loop *:
<?php
if( $field = get_post_meta( get_the_ID(), 'cart_key', true ) ) {
?>
<a href="<?php echo $field; ?>" id="btn-book-now" class="btn-custom">
<?php echo __('Purchase Now','Vierra'); ?>
</a>
<?php
}
?>
* Otherwise, you'd use $post->ID instead of get_the_ID().