Because it's returning 0 for that string, which evaluates to false. Strings are zero-indexed and as such if http://
is found at the beginning of the string, the position is 0, not 1.
You need to compare it for strict inequality to boolean false using !==
:
if(strpos($URL, "http://") !== false)
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…