Substr is a function which have two parameter accepted. The first one is a string, the second is the start of where you want to index the position where you want to cut the string. The last one is the length which is optional if the value of start parameter is less than 0 is considered like where you want to cut your string. Here's how it work:
echo substr("string", 1, 3); // return "tri" because it's indexed
In the instruction, it tells you to play around with these strings. Here's what you started:
See, rand function only parameter accepted is an integer and your substr will print a part of a string. Plus, it asks you to print a random character inside of your name and rand function will print a number. So you need to change. I can't print out the code for you otherwise that will be cheating (Community Guidelines) but I can give you examples and explanation of how to do it.
You can first of all use the substr function which will print a string (that's what we want! + your name surely doesn't contain any number in it!). In the first parameter, you will input your name since it only accepts strings. In second and third parameter, use rand and strlen function to define the start and length parameter. Remember that start parameter should not have a negative value otherwise it will return false!