If you want to be more flexible, there is the sprintf function which does the same thing as printf but returns a string. Also, you can put the format into a variable for consistency and flexibility across your program.
$fmt = "%20.18f";
print sprintf( $fmt, 2.19502183680891e-06 )."\n";
For every complex problem there is an answer that is clear, simple, and wrong. H. L. Mencken