Questions about PHP characters and arrays

Questions about PHP characters and arrays $ str = "123456"; var_dump ($ str ["abc"]); why is the result the first character instead of null & nbsp; in addition, when I unset ($ str ["abc"]), the system prompts that this subscript does not exist ., The character string is not an array and has only a position. no subscript or associated key array functions cannot be used as strings. Therefore, & nb's questions about PHP characters and arrays
$ Str = "123456 ";
Var_dump ($ str ["abc"]);

Why is the result the first character instead of null and I am unset ($ str ["abc"]) again prompting that this subscript does not exist.
------ Solution --------------------
A string is not an array and only has a position. it has no subscript or associated key.
Array functions cannot be used as strings.

Therefore, "abc" in $ str ["abc"] is converted to a value of 0.
For $ str = "123456"; $ str ["abc"] is $ str [0], so 1 is returned.

Although you can write $ str [$ I] at a location of the access string
To prevent ambiguity, we recommend that you write $ str {$ I} like this in php}
------ Solution --------------------
Manual comparison operators
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Var_dump (0 = "a"); // 0 = 0-> true
Var_dump ("1" = "01"); // 1 = 1-> true
Var_dump ("1" = "1e0"); // 1 = 1-> true
Switch (""){
Case 0:
Echo "0 ";
Break;
Case "a": // never reached because "a" is already matched with 0
Echo "";
Break;
}
?>
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The switch example is obvious.

The string index cannot use a "string", but php intelligently converts it to 0.