Can you please give an example of what you might be putting into the column? With mediumtext, I dont believe you have to specify a length.
But I think what your problem is, is the way you have serialized to be inserted.
'". $sqlinput ."' Is weird. Even if it may be correct by php standards(I never do it, but I dont wanna say its invalid syntax), you generally only want to have the 2 single quotes on both sides of the variable. And I dunno why you have the dots on both sides also. So make it:
"$sqlinput"
And that should work.