在为数组赋值时，有没有理由在PHP中使用=>而不是=？

Please note that I have already read Reference — What does this symbol mean in PHP? and What Does This Mean in PHP -> or => and I know that what => do in PHP.

My question is different.

Generally most programming languages use = to assign value to another.

Example 01

$my_name = "I am the Most Stupid Person"; //Yes, that is my name in SO :-)

Example 02

$cars = array();

$cars[0] = "Volvo";
$cars[1] = "BMW";
$cars[2] = "Toyota";

Now let see following example.

$myArray = array(
    0 => 'Big',
    1 => 'Small',
    2 => 'Up',
    3 => 'Down'
);

Here is also what happen is we have assigned 'Big' for $myArray['0'].

But here we used => instead of =. Is there any special reason that PHP was designed that way?

Consistency of syntax is important, here I would say using => in arrays is to ensure = still works. For example:

$a = 5;

Sets the variable $a to 5.

$a = $b = 5;

Sets the variable $a and $b to 5. That is = as an operator assigns the right hand side to the left hand side (if possible) and its result is also the right hand side. So now, in the context of an array:

$a = array(
    0 => 'foo'
  );

Now $a[0] is 'foo'.

$a = array(
    0 => $b = 'foo'
  );

Now $a[0] and $b are both 'foo'. Now think about this:

$b = 0;
$a = array(
    $b => 'foo'
  );

Simply means $a[$b], that is, $a[0] is 'foo'. If PHP used = for array keys:

$b = 1;
$a = array(
    $b = 'bar'
  );

What is the value of $a? Is it [1 => 'bar']? Or is it [0 => 'bar']? Did $b get the value 'bar'? Or was it only used as a key?

As you can see, the parser would be very confusing this way, and there would be no way to allow keys defined by variables with this syntax.

It's because parser is written in that way.

Also it's more readable than $variable = "", witch means $variable now has value "" instead of 'key' => "" as In array at position 'key' there is value ""