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Looks like php4 implicit declarations inside 
Another possibility could indeed be to set $autoresize to true just before if ($validPost) (your last else could more or less be seen as the default case when using switch instead of if), so this value would be overridden by any false given to it after.
But the problem comes apparently just from one of my blocker, as the concerned checkbox is indeed checked when I use a "standard user" profile in my browser or another browser without blocker. I'll make tests some day to understand which one is responsible, but it's not that troublesome: I only "clean up" my browser once a month
Another possibility could indeed be to set $autoresize to true just before if ($validPost) (your last else could more or less be seen as the default case when using switch instead of if), so this value would be overridden by any false given to it after.
But the problem comes apparently just from one of my blocker, as the concerned checkbox is indeed checked when I use a "standard user" profile in my browser or another browser without blocker. I'll make tests some day to understand which one is responsible, but it's not that troublesome: I only "clean up" my browser once a month