Question

href=" <?php 
$zzz_stylesheet = "http://127.0.0.1/www/wordpress/wp-content/themes/mytheme/style.css"; 
echo str_replace(".css","-mytheme.css",$zzz_stylesheet);
?>

works but

href=" <?php 
$zzz_stylesheet = bloginfo('stylesheet_url'); 
echo str_replace(".css","-mytheme.css",$zzz_stylesheet);
?>

does not work. Why?
bloginfo is a function in WordPress which returns the value I have used in first code line.

Answer 1

From WordPress’ documentation to bloginfo:

Displays information about your blog […] This always prints a result to the browser. If you need the values for use in PHP, use get_bloginfo().

(emphasis added)

So: Use get_bloginfo instead.