How can create function like this in PHP? [closed]

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I want to create function like this. please help me to complete my function if both are +ve

$a = 10;
$b = 6;
// do nothing just show same value 10 and 6 
// if $b is negetive
$a = 10;
$b = -3;
// get the value from $a show the value 7 (10 - 3)
// 0 if $a is negative 
$a = -2;
$b = 6;
// (little bit diffrent) get the double form $b of negative value and
// show the value 0 and 2(6-4)

i have tried with this but totally stopped at else code

// $a = something;
// $b = something;

if ($a >0 && $b >0) {
  echo $a.' , '.$b;
}else{ 
  // what? what? what?
} 

---

Something along the lines of

<?php
if ($a > 0) {
    if ($b > 0) echo $a." ".$b;
    else echo ($a+$b)." 0";
}
else {
    if ($b > 0) echo "0 ".($b+$a*2);
    //else undefined?
}
?>

Looks a bit homeworky though