循环变量值，

I have a variable generated by $_POST called $quantity,

What would be the correct way to run a loop based on the value of this variable?

Well, assuming that $quantity is numerical, you can do this:

for ($c = 0; $c < $quantity; $c++) {
  //do whatever
}

You can do a for loop (I'm assuming you want to run some code $quantity number of times)

for ($i = 0; $i < $quantity; $i++) {
    // 
}