php 5 strpos() difference between returning 0 and false?

Yes, this is correct / expected behavior :

• strpos can return 0 when there is a match at the beginning of the string
• and it will return false when there is no match

The thing is you should not use == to compare 0 and false ; you should use ===, like this :

if(strpos("abcdefghijklmnop","http://www.") === 0) {}

Or :

if(strpos("abcdefghijklmnop","http://www.") === false) {}

For more informations, see Comparison Operators :

• $a == $b will be TRUE if $a is equal to $b.
• $a === $b will be TRUE if $a is equal to $b, and they are of the same type.

And, quoting the manual page of strpos :

This function may return Boolean FALSE, but may also return a non-Boolean value which evaluates to FALSE, such as 0 or "".
Please read the section on Booleans for more information.
Use the === operator for testing the return value of this function.

=== and !== compare type and value as shown below:

if (strpos("abcdefghijklmnop", "http://www.") !== false) {     // do work     }

strpos returns an int or boolean false. the == operator also evaluates 0 to mean false, you want to use the === operator (three equals signs) that also checks that the types being compared are the same instead of just seeing if they can be evaluated to mean the same.

so

if (strpos($hastack, $needle) === 0){    // the $needle is found at position 0 in the $haystack}