php loop flip coin until 2 heads, then stop - php

Question

I'm new to php and am trying to write a loop that will flip a coin until exactly two heads have been flipped and then stop.
So far I've written a function for coin flipping:
function cointoss () {
$cointoss = mt_rand(0,1);
$headsimg = '<img src=""/>';
$tailsimg = '<img src=""/>';
if ($cointoss == 1){
print $headsimg;
} else {
print $tailsimg;
}
return $cointoss;
}
...but am stuck on writing the loop. I've tried a couple ways:
#this code takes forever to load
$twoheads = 0;
for ($twoheads = 1 ; $twoheads <= 20; $twoheads++) {
$cointoss = mt_rand(0,1);
cointoss ();
if ($cointoss == 1) {
do {
cointoss ();
} while ($cointoss == 1);
}
}
#one coin flips
do {
cointoss ();
} while ($cointoss == 1);
This is a for a class, and we haven't learned arrays yet, so I need to accomplish this without them.
I understand the concept of loops executing code while a condition is true, but don't understand how to write for when a condition is no longer true.

Answer 1

Printing from inside of "processing functions" is a bad habit to get into. You might like to declare a showCoin($toss) function for printing. In truth, I don't know if I would bother with any custom functions.
You need to declare a variable which will hold the return value from your function.
By storing the current and previous toss values, you can write a simple check if two consecutive "heads" have occurred.
Code: (Demo)
function cointoss () {
return mt_rand(0,1); // return zero or one
}
$previous_toss = null;
$toss = null;
do {
if ($toss !== null) { // only store a new "previous_toss" if not the first iteration
$previous_toss = $toss; // store last ieration's value
}
$toss = cointoss(); // get current iteration's value
echo ($toss ? '<img src="heads.jpg"/>' : '<img src="tails.jpg"/>') , "\n";
// ^^^^^- if a non-zero/non-falsey value, it is heads, else tails
} while ($previous_toss + $toss != 2);
// ^^^^^^^^^^^^^^^^^^^^^^- if 1 + 1 then 2 breaks the loop
Possible Output:
<img src="heads.jpg"/>
<img src="tails.jpg"/>
<img src="tails.jpg"/>
<img src="tails.jpg"/>
<img src="heads.jpg"/>
<img src="heads.jpg"/>