verify, if the string starts with given substring

You should check with the identity operator (===), see the documentation.

Your test becomes:

if (strpos($str, 'http://') === 0) echo 'yes';

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As @Samuel Gfeller pointed out: As of PHP8 you can use the str_starts_with() method. You can use it like this:

if (str_starts_with($str, 'http://')) echo 'yes';

You need to do:

if (strpos($str, "http://") === 0) echo "yes"

The === operator is a strict comparison that doesn't coerce types. If you use == then false, an empty string, null, 0, an empty array and a few other things will be equivalent.

See Type Juggling.

PHP does have 2 functions to verify if a string starts with a given substring:

• strncmp (case sensitive);
• strncasecmp (case insensitive);

So if you want to test only http (and not https), you can use:

 if (strncasecmp($str,'http://',7) == 0) echo "we have a winner"