echo generates the dreaded " Warning: A non-numeric value encountered..."

Question

This Warning message results from a simple echo of a string concatenated with a simple arithmatic expression...

$x = 21.12345;
echo "x=" . $x ; // fine
echo "<hr>";
$y = $x + 5;
echo "y=" . $y; //OK so far
echo "<hr>";
echo 'stringMeUp' . $x + 21 ; // generates the warning
echo "stringMeUp" . (float)$x + (int)21 ; // generates the warning
echo "<hr>";

Any advice greatly appreciated.