PHP - Find if a string contains only whitespaces and nothing else

Question

How do I find out if a string contains ONLY whitespaces and nothing else? The number of such spaces does not matter.

if () // $string contains ONLY whitespaces and nothing else   
{
// do something
}

Best if can provide a solution that can be generalised to any character: How do I find if a string contains ONLY some character and nothing else

Answer 1

A non-regex approach could be:

if(strlen($string) >= 1 && empty(trim($string))) {

empty(trim($string)) removes all whitespace then checks if the string is empty. strlen($string) >= 1 confirms the string wasn't empty to start with. If empty strings are also allowed that check can be removed.