Srcset php

gistfile1.txt

<?php 
define( 'MEDIUM_BREAKPOINT', '64em' );
define( 'MEDIUM_BREAKPOINT_CONTENT_WIDTH', '674px' );

$post_thumbnail_id = get_post_thumbnail_id();

if ( $post_thumbnail_id ) :
	$landscape_feature_retina = wp_get_attachment_image_src( $post_thumbnail_id, 'landscape-feature-retina' );
	$landscape_feature_med    = wp_get_attachment_image_src( $post_thumbnail_id, 'landscape-feature-med' );
	$landscape_feature_sm     = wp_get_attachment_image_src( $post_thumbnail_id, 'landscape-feature-sm' );
	$alt_text                 = get_post_meta( $post_thumbnail_id, '_wp_attachment_image_alt', true );
	?>

	<img src="<?php echo esc_url( $landscape_feature_med[0] ); ?>" 
		srcset="<?php echo esc_url( $landscape_feature_retina[0] ); ?> <?php echo absint( $landscape_feature_retina[1] ); ?>w,
				<?php echo esc_url( $landscape_feature_med[0] ); ?> <?php echo absint( $landscape_feature_med[1] ); ?>w,
				<?php echo esc_url( $landscape_feature_sm[0] ); ?> <?php echo absint( $landscape_feature_sm[1] ); ?>w" 
		sizes="(min-width: <?php echo esc_attr( MEDIUM_BREAKPOINT ); ?>) <?php echo esc_attr( MEDIUM_BREAKPOINT_CONTENT_WIDTH ); ?>,
			   100vw" 
	 	alt="<?php echo esc_attr( $alt_text ); ?>"
	 	class="featured-image" />

<?php endif; ?>