PHP Arrays: How to add 'key & value' to an existing array - php

Question

I do not know how to add a key and value to the existing array. My array goes like this. Initially I have tried adding using array_push() but it added not as I needed it.
I have given my output after I gave the 'var_dump'.
array (size=5)
0 =>
array (size=3)
'id' => int 7
'title' => string 'Pongal' (length=6)
'start' => string '2016-05-16' (length=10)
1 =>
array (size=3)
'id' => int 8
'title' => string 'big day' (length=7)
'start' => string '2016-05-04' (length=10)
2 =>
array (size=3)
'id' => int 9
'title' => string 'marriage day' (length=12)
'start' => string '2016-05-19' (length=10)
3 =>
array (size=3)
'id' => int 10
'title' => string 'Karthiks bday' (length=14)
'start' => string '2016-06-11' (length=10)
4 =>
array (size=3)
'id' => int 12
'title' => string 'Election date announced' (length=23)
'start' => string '2016-06-01' (length=10)
Now, I'd like to insert array('sample_key' => 'sample_value') after all the elements of each array.
How can I do it? This is I want the result to be like this:-
array (size=5)
0 =>
array (size=4)
'id' => int 7
'title' => string 'Pongal' (length=6)
'start' => string '2016-05-16' (length=10)
‘color’ => ‘red’
1 =>
array (size=4)
'id' => int 8
'title' => string 'big day' (length=7)
'start' => string '2016-05-04' (length=10)
‘color’ => ‘red’
2 =>
array (size=4)
'id' => int 9
'title' => string 'marriage day' (length=12)
'start' => string '2016-05-19' (length=10)
‘color’ => ‘red’
3 =>
array (size=4)
'id' => int 10
'title' => string 'Karthiks bday' (length=14)
'start' => string '2016-06-11' (length=10)
‘color’ => ‘red’
4 =>
array (size=4)
'id' => int 12
'title' => string 'Election date announced' (length=23)
'start' => string '2016-06-01' (length=10)
‘color’ => ‘red’
Note that I have added 'color' => 'red' to all the indexes

Answer 1

Just do this: Working demo
using the & you can change the main array, and just use $val['color'] = 'red' to add a new key , value pair in the array.
foreach($arr as $key => &$val){
$val['color'] = 'red';
}
Note that the 'write-back' feature of the ampersand persists even after the loop has finished: resetting $val to a new value will change the last element in $val, which is often unexpected. There are three ways around this class of bug:
Avoid write-back and just use the full array expression to write values inside the loop;
Don't re-use the $val variable in the same scope, even for another foreach() loop;
Use unset() on the $val variable to disconnect it from the array it will write back to.

Answer 2

foreach($arr as $key => $row){
$arr[$key]['color']="red";
}