function get_contents($function) {
    ob_start();
    $function();
    $contents = ob_get_contents();
    ob_end_clean();
 
    return $contents;
}

// Example: get function's response to variable.
function add($this, $that) {
    echo $this . ' + ' . $that . ' = ' . ($this + $that);
}
 
$example = get_contents(function() {
    add(1, 1);
});
 
echo $example . "\n"; // 1 + 1 = 2

// Example 2: get function's response using return. Even if the function echos output, it can be captured to a variable.
// Compare print_r() that has a return parameter to var_dump() that has no such parameter.
 
$return = true;
$example2 = print_r(array('a', 'b', 'c'), $return);
 
// Solution: use get_contents() to capture the function's response to a variable.
$example3 = get_contents(function() {
    var_dump(array('a', 'b', 'c'));
});
 
echo $example3;
// array(3) {
//   [0]=>
//   string(1) "a"
//   [1]=>
//   string(1) "b"
//   [2]=>
//   string(1) "c"
// }