$a = "abcd";
$b = "1234";
echo "初始化时 a=$a,b=$b<br>";
$a = $a . $b;
$b = strlen( $b );
$b = substr( $a, 0, (strlen($a) - $b ) );
$a = substr( $a, strlen($b) );
echo "交换后 a=$a,b=$b<br>";

------解决方案--------------------
php已经提供了绝好的途径

$a = "abcd";
$b = "1234";

list($b, $a) = array($a, $b);

echo "$a $b"; //1234 abcd