wp get meta field value

<?php $key_1_value = get_post_meta( get_the_ID(), 'key_1', true ); ?>

<?php if ( get_post_meta( get_the_ID(), 'thumb', true ) ) : ?>
    <a href="<?php the_permalink() ?>" rel="bookmark">
        <img class="thumb" src="<?php echo esc_url( get_post_meta( get_the_ID(), 'thumb', true ) ); ?>" alt="<?php the_title_attribute(); ?>" />
    </a>
<?php endif; ?>