Simple substring implementation

Substring

//Simple substr implementation

//Paramaters
//String:the string you want to operate on
//Start:starting position, indexed from 0
//End:ending position, indexed from 0
//Curpos:used internally by the recursion loop- leave blank

//Examples
//echo(substr("Hello world",0,4)); Returns "Hello"
//echo(substr("Hello world",6,10)); Returns "world"

//If start/end are out of range from the string, those values will show up as undef in the resulting string

function substr(string,start,end,curpos=0)=(start+curpos < end)?str(string[start+curpos],substr(string,start,end,curpos+1)):string[start+curpos];