Multiple blank lines with spaces, tabs and new lines to one line?

I know there are similar questions, but i have certain different problems!

I need to convert multiple blank lines to o开发者_如何学JAVAne in PHP, but when I use not only New Lines, but Spaces and New Lines it doesn't work. If there are 2 or more blank lines(with spaces and enter's), they should be converted in only one blank line, when it's only new line(but not blank line), it should remain the same.

I used this, but it doesn't work with spaces and new lines:

$string = str_replace(array("\r\n", "\r", "\n", "<br />", $string);

echo preg_replace("~<br />(<br\s*/?>[\r\n]*)+~i", "<br /><br />", $string);

I also need it to be displayed in this way in Windows, Linux and Mac!

I think these are the only things you need

$str = str_replace(array("\n", "\r", "\t"), ' ', $str);
$str = preg_replace('/(\s+)/', ' ', $str);

Try following

<?php 
$str = ' 



aa 
a 


a 

a 
'; 

for($i = 1;$i< 8;$i++){ 
    $str = str_replace("\r\n\r\n\r\n", "\r\n\r\n", $str); 
    $str = str_replace("\t", '', $str); 
} 

echo $str;

try this

preg_replace("/(^[\r\n]*|[\r\n]+)[\s\t]*[\r\n]+/", "\n", $string);