By using the PHP implode() function you can easily add items to an array and then output them as a single string.
// Create initial array of CSS classes
$css_classes = array('btn');
// If we're on the blog page (is_home() is a WordPress function)
if(is_home()) {
$css_classes[] = 'btn--large';
$css_classes[] = 'btn--wide';
}
// Convert array to string
$css_classes = implode(' ', $css_classes);
<!-- Function in use -->
<a class="<?php echo $css_classes; ?>" href="#">Button!</a>
<!-- Output -->
<a class="btn btn--large btn--wide" href="#">Button!</a>
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