Doubts about PHP characters and arrays

Questions about PHP characters and arrays
$str = "123456";
Var_dump ($str ["abc"]);

Why the result is the first character and not the null and I unset ($str ["ABC"]) and I'm not suggesting that subscript.
------Solution--------------------
String is not an array, only position, no subscript and association key
Array-related functions cannot be used for strings

So the "ABC" in $STR ["ABC"] is converted to a value of 0
For $str = "123456"; In terms of $STR ["ABC"] is $str [0], so get 1

Although you can write a location that accesses a string $str [$i]
But to avoid ambiguity, PHP suggests you write this $str {$i}
------Solution--------------------
Manual comparison Operators Section
------------------------------------------
Var_dump (0 = = "a"); 0 = = 0-True
Var_dump ("1" = = "01"); 1 = = 1-True
Var_dump ("1" = = "1e0"); 1 = = 1-True
Switch ("a") {
Case 0:
echo "0";
Break
Case ' a ':///Never reached because "a" is already matched with 0
echo "a";
Break
}
?>
------------------------
Switch, the example is obvious.

The string index cannot be "string", but PHP handles it "intelligently" to become 0

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