I am trying to convert some old library functions into a class. (i'm trying to force myself to learn OOP)

The Class:
PHP Code:
class timestamp {

    protected 
$input;
    protected 
$aDate;
    protected 
$datetime;
    
    public function 
__construct($input) {
        
$this->input $input;
    }
    
    public function 
stamp() {
     
        if(
strpos($this->input"-") && !strpos($this->input":")) {
                  
$aDate explode("-",$this->input);
                
$datetime mktime(000$aDate[1], $aDate[2], $aDate[0]);
        }
        elseif(
strpos($this->input":") && !strpos($this->input"-")) {
                  
$aTime explode(":",$this->input);
                
$datetime mktime($aTime[0], $aTime[1], $aTime[2], 112007);
        }
        else {
                  
$datetime explode(" ",$this->input);    
                
$aDate explode("-",$datetime[0]);
                
$aTime explode(":",$datetime[1]);
                
$datetime mktime($aTime[0], $aTime[1], $aTime[2], $aDate[1], $aDate[2], $aDate[0]);
            }
    return 
$datetime;
    }

calling the class produces the error:

Catchable fatal error: Object of class timestamp could not be converted to string
I've worked out its the strpos() function that's causing the error and I understand that you can use __toString() to convert an object to a string but am not sure how to implement it in this set-up.

Any help much appreciated.