Adding element to a multidimensional array in PHP - php

Question

I'm iterating over an array of courses, which all contain category IDs in the form of strings. I'm trying ro group them all by category using a matrix, but I'm having trouble doing that.
Here's an example of what I want:
$courses_by_code = [[2/11]=>[course1],[course2]], [[3/12]=>[course3], [course4]]
So my questions are:
How do I add an element that has a key that's already in the matrix to the array that corresponds to that key?
How do I create a new line in the matrix in case I find a new key?

Answer 1

I am not sure I understood 100%, but from what I understood you should do something like:
$keyThatMightExist // '2/11' for example
if(isset($courses_by_code[$keyThatMightExist])) {
$courses_by_code[$keyThatMightExist][] = $newCourseToAdd;
} else {
$courses_by_code[$keyThatMightExist] = array($newCourseToAdd);
}

Answer 2

Let's start with PHP's Arrays documentation:
An array in PHP is actually an ordered map. A map is a type that associates values to keys.
Something that will be invaluable to you when working with arrays is the print_r function. var_dump is also useful. You will be able to see array structure as its stored in PHP. Here's some useful things to know with arrays.
Let's answer some questions now:
How do I add an element that has a key that's already in the matrix to the array that corresponds to that key? How do I create a new line in the matrix in case I find a new key?
$courses = []; // initialize
$courses['2/11'][] = 'course1';
$courses['2/11'][] = 'course2';
$courses['3/12'][] = 'course3';
$courses['3/12'][] = 'course4';
The empty [] you see indicates that I'm adding more elements to the key 2/11. If I wanted I could also name those keys, but I'm not going to do that. Using print_r (described above) I will now print the array out in a human-readable format. Note that with print_r you generally want to surround the output with <pre> tags, as such:
echo "<pre>";
print_r($courses);
echo "</pre>";
And here is my output:
Array
(
[2/11] => Array
(
[0] => course1
[1] => course2
)
[3/12] => Array
(
[0] => course3
[1] => course4
)
)
Here is how I can access these elements:
echo $courses['2/11'][1]; // this is course2
echo "<br>";
print_r($courses['3/12']); // this is the entire '3/12' array
Result:
course2
Array
(
[0] => course3
[1] => course4
)