WARNING

As strpos may return either FALSE (substring absent) or 0 (substring at start of string), strict versus loose equivalency operators must be used very carefully.

To know that a substring is absent, you must use:  

=== FALSE

To know that a substring is present (in any position including 0), you can use either of:

!== FALSE  (recommended)
 > -1  (note: or greater than any negative number)

To know that a substring is at the start of the string, you must use:  

=== 0

To know that a substring is in any position other than the start, you can use any of: 

 > 0  (recommended)
!= 0  (note: but not !== 0 which also equates to FALSE)
!= FALSE  (disrecommended as highly confusing)

Also note that you cannot compare a value of "" to the returned value of strpos. With a loose equivalence operator (== or !=) it will return results which don't distinguish between the substring's presence versus position. With a strict equivalence operator (=== or !==) it will always return false.