The space ofcourse.
It seems a small step from this:
$sOne = 'this';
$sTwo = 'that';
echo $sOne.$sTwo;
To this:
$sOne = 'this';
$sTwo = 'that';
$sSpace = ' ';
echo $sOne.$sSpace.$sTwo;
To this:
$sOne = 'this';
$sTwo = 'that';
echo $sOne.' '.$sTwo;
Or even simpler:
$sOne = 'this';
$sTwo = 'that';
echo $sOne;
echo ' ';
echo $sTwo;