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

if(strpos("http://www.example.com","http://www.")==0){ // do work}

I'd expect this to resolve as true, which it does. But what happens when I do

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

This also passes on php 5 because as far as I can work out the strpos returns false which translates as 0.

Is this correct thinking/behaviour? If so what is the workaround for testing for that a substring is at the beginning of another string?

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
}

0 is a possible return value from strpos when it finds a match at the very beginning. In case if the match is not found it returns false(boolean). So you need to check the return value of strpos using the === operator which check the value and the type rather than using == which just checks value.