This markup is generated on the js side of things and saved in the content of the block editor, which is why there doesn’t seem to be a native PHP function for this.
However, I found a PHP method that does exactly this in an “experimental” class in the Gutenberg plugin. You can see this here: https://github.com/WordPress/gutenberg/blob/master/lib/class-experimental-wp-widget-blocks-manager.php#L265
You could add it as a method in your own class or convert to a standard function like so:
/**
* Serializes a block.
*
* @param array $block Block object.
* @return string String representing the block.
*/
function serialize_block( $block ) {
if ( ! isset( $block['blockName'] ) ) {
return false;
}
$name = $block['blockName'];
if ( 0 === strpos( $name, 'core/' ) ) {
$name = substr( $name, strlen( 'core/' ) );
}
if ( empty( $block['attrs'] ) ) {
$opening_tag_suffix = '';
} else {
$opening_tag_suffix = ' ' . json_encode( $block['attrs'] );
}
if ( empty( $block['innerHTML'] ) ) {
return sprintf(
'<!-- wp:%s%s /-->',
$name,
$opening_tag_suffix
);
} else {
return sprintf(
'<!-- wp:%1$s%2$s -->%3$s<!-- /wp:%1$s -->',
$name,
$opening_tag_suffix,
$block['innerHTML']
);
}
}