there is no warning because there is actually nothing to warn about. when you say $foo = $bar you're just assigning the value and there will be no conditions to evaluate. Inside an `if' conditional, the value assigned to the LHS will be used as is, like

$bar = 2;
if($foo = $bar) {
# do something
}
[download]

is same as

if(2){
# do something
}
[download]