In the PHP manual, (array_push) says..
If you use array_push() to add one element to the array it's better to
use $array = because in that way there is no overhead of calling a
$arr = array();
$arr = "stackoverflow";
When you call a function in PHP (such as
array_push()), there are overheads to the call, as PHP has to look up the function reference, find its position in memory and execute whatever code it defines.
$arr = 'some value'; does not require a function call, and implements the addition straight into the data structure. Thus, when adding a lot of data it is a lot quicker and resource-efficient to use