How do you check if a string starts with a substring in php?

I have a string in $str variable.

How can I verify if it starts with some word?

Example:

$str = "//somesite.com/somefolder/somefile.php";

When I wrote the following script returns yes

if[strpos[$str, "//"] == '0'] echo "yes";

BUT it returns yes even when I wrote

if[strpos[$str, "other word here"] == '0'] echo "yes";

I think strpos returns zero if it can't find substring too [or a value that evaluates to zero].

So, what can I do if I want to verify if word is in the start of string? Maybe I must use === in this case?

All strings start with the empty string