How do I remove blank lines from text in PHP?

I need to remove blank lines (with whitespace or absolutely blank) in PHP. I use this regular expression, but it does not work:

$str = ereg_replace('^[ \t]*$\r?\n', '', $str);
$str = preg_replace('^[ \t]*$\r?\n', '', $str);

I want a result of:

blahblah

blahblah

   adsa 


sad asdasd

will:

blahblah
blahblah
   adsa 
sad asdasd