Testing for $_POST var affects variable even though $_POST should be undefined

[CurseDoX]

I have used for years this method of testing for a $_POST var and assigning a value to $var simultaneously. Today I ran into a server that seems to be different in its handling of this.I'm loading the page http://site.com/foo.php?var=bar No form post made... and testing for $_POST var seems to unset my initial assignment \[code\]$var= $_GET['var'];echo "var is $var\n"; // correctly echos $_GET['var'] if ($var= $_POST['foo']) { echo "POST foo seen\n"; // does not fire, condition not met}echo "var is $var\n"; // does not echo $_GET['var'] \[/code\]When I switched to \[code\]if (isset($_POST['foo'])) { $var = $_POST['foo'];...\[/code\]I got rid of the unsetting behavior.So my question is whether this \[code\]if ($var= $_POST['foo']) {\[/code\] is bad form?