Do you just mean spaces or all whitespace?
For just spaces, use str_replace:
$string = str_replace(' ', '', $string);
For all whitespace (including tabs and line ends), use preg_replace:
$string = preg_replace('/\s+/', '', $string);
(From here).
str_replace will do the trick thusly
$new_str = str_replace(' ', '', $old_str);
If you know the white space is only due to spaces, you can use:
$string = str_replace(' ','',$string);
But if it could be due to space, tab...you can use:
$string = preg_replace('/\s+/','',$string);
If you want to remove all whitespace:
$str = preg_replace('/\s+/', '', $str);
See the 5th example on the preg_replace documentation. (Note I originally copied that here.)
Edit: commenters pointed out, and are correct, that str_replace is better than preg_replace if you really just want to remove the space character. The reason to use preg_replace would be to remove all whitespace (including tabs, etc.).
Source: Stackoverflow.com