substr return empty string - php

Question

i have problem with $length of substr function
my CODE
$string='I love stackoverflow.com';
function arabicSubStr($value,$start,$length=false){
return mb_substr($value,$start,$length,'UTF-8');
}
echo arabicSubStr($string,7);//outputs nothing
echo substr($string,7);//outputs stackoverflow.com
The reason of the problem is:
If length is given and is 0, FALSE or NULL an empty string will be returned.
So, How i can fix the problem?
i won't use strlen($string)
EDITE
I know the reason is because i've defined $length as false
And i am here to know what should i put in $length parameter to avoid this error?
i am trying to put -1 it's returns //stackoverflow.co

Answer 1

Since the reason you're getting an empty string is specified entirely by the content of your question (using 0, FALSE or NULL), I assume you just want a way to get the rest of the string.
In which case, I'd use something like:
function arabicSubStr ($value, $start, $length = -1) {
if ($length == -1)
$length = mb_strlen ($value, 'UTF-8') - $start;
return mb_substr ($value, $start, $length, 'UTF-8');
}
You need to do it this way since there is no sentinel value of length that means "the rest of the string". Positive numbers (and zero) will limit the size to that given, negative numbers will strip off the end of the string (as you show in your question edit).
If you really don't want to use a string length function, you could try a value of 9999 (or even higher) and hope that:
the mb_substr() function will only use it as a maximum value; and
you won't pass in any strings 10K or more.
In other words, something along the lines of:
function arabicSubStr ($value, $start, $length = 9999){
return mb_substr ($value, $start, $length, 'UTF-8');
}
Though keep in mind I haven't tested that, I don't have any PHP environments at my current location.

Answer 2

It's because you have $length set to false as the default parameter for your function, which effectivley means you want it to return a substring of 0 length.
Unfortunately, if you have to set the final parameter (the charset) which I imagine you do, then you have to calculate the length of the string first, so something like:
function arabicSubStr($value,$start,$length=false){
$length = ($length) ? $length : mb_strlen($value,'UTF-8') - $start;
return mb_substr($value,$start,$length,'UTF-8');
}