strStr and strPos are practically the same thing. They both find the location of a string inside of another string.
Example: strStr("Hello", "he"); will return 0, which is the position of the first charachter of the string. strPos will return the same thing.
Arguments:
strStr(string, string to find inside of string);
strPos(string, string to find inside of string);
StrLen gets the length of a string.
Example: strLen("Hello"); will return 5, which is the number of charachters in the string.
Arguments:
strLen(string);