(PHP) struggling with the variable inside a for loop

To give some context, this is the code that the for loop refers to: putting the form data into an array.

$_SESSION['shoppingcart'][$count] = array (

'id' => filter_input(INPUT_GET, 'ID'),
'name' => filter_input(INPUT_POST, 'hidden_name'),
'price' => filter_input(INPUT_POST, 'hidden_price'),
'list_price' => filter_input(INPUT_POST, 'hidden_list_price'),
'quantity' => filter_input(INPUT_POST, 'quantity')

);

$count = count($_SESSION['shoppingcart']);

$product_ids = array_column($_SESSION['shoppingcart'], 'id');


This is all well and good: I understand what this code does. It's just the 'for' loop and everything that involves the variable '$i' outside of the for loop.

for ($i=0; $i < count($product_ids); $i++) {

if ($product_ids[$i] == filter_input(INPUT_GET, 'ID')) {

$_SESSION['shoppingcart'][$i]['quantity'] +=
filter_input(INPUT_POST,'quantity');

}
}


So, the $i variable. What does it represent in the for loop and why is it/ what does it represent in square brackets in the 'if' statement.

$i

I should say that this code accomplishes its intended purpose (putting form data into an array, incrementing the quantity if the item in the array already exists.)
I got the code from a YouTube tutorial, only he didn't explain the $i part.

quantity

$i

2 Answers
2

$i is a variable which is set to 0 by $i=0 and which has its value increased by 1 in the third part of for loop (i++). The for loop works while $i is smaller than the amount of values in $product_ids array. Therefore, if there are 20 values in $product_ids this loop will execute the code 20 times, but if you changed $i++ what is a short code of $i=$i+1 to $i = $i + 2 it would execute the code between {} 10 times and value of $i would change like this: 0, 2, 4 ... 18, 20.

$i

$i=0

i++

$i

$product_ids

$product_ids

$i++

$i=$i+1

$i = $i + 2

{}

$i

brackets in if shows that $product_ids is an array and allows to select specific value (in this code that value is $i meaning that it will change every time code executes (as $i as, I said, increases from 0 to count of values in $product_ids).

if

$product_ids

$i

$i

$product_ids

Is it clear now?

Everytime you complete an iteration of your loop the $i variable will increase by $i++ (thus in this case by 1) until this condition: $i < count($product_ids) is met. This is particularly useful for accessing elements in an array sequentially, namely if ($product_ids[$i] == filter_input(INPUT_GET, 'ID')) is used to access each element of $product_ids and check if that value is equal to filter_input(INPUT_GET, 'ID').

$i

$i++

$i < count($product_ids)

if ($product_ids[$i] == filter_input(INPUT_GET, 'ID'))

$product_ids

filter_input(INPUT_GET, 'ID')

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