Questions about double quotation marks, plus signs, and periods

I want to output $ a = 1 in double quotes, plus signs, and periods on the screen, so I wrote the following, but only the last one is correct. If you have two questions, the first one is why the first two outputs only numbers. What are the preceding & quot; $ a = & quot; and '$ a =? The second point is that the plus sign and the dot represent the double quotation marks, the plus sign, and the DOT sign in PHP.
I want to output $ a = 1 on the screen, so I wrote the following, but only the last one is correct.
I have two questions,
The first point is why the first two outputs only numbers. how does the former "$ a =" and '$ a =' handle them?
What exactly does the plus sign and the dot represent in PHP? Why is no error reported when the plus sign is used for the first two?
$ A = 1;
Echo "$ a =" + $ a; // error 2
Echo '$ a =' + $ a; // error 1
Echo "$ a =". $ a; // error 1 = 1
Echo '$ a ='. $ a; // correct $ a = 1
?>

I just got in touch with PHP and read the materials. I didn't find these two knowledge points. so I am going to ask for help. I hope you will be enlightened by the PHP experts. Thank you!

------ Solution --------------------
Variables are parsed in double quotes, but not in single quotes. . Is the string connector, + is the addition
------ Solution --------------------
+ Arithmetic operators
. String connector

------ Solution --------------------
Php will parse the string, "" will parse the internal escape or $, ''will only escape the internal \ ', regardless of other.


Echo "$ a =" + $ a; is because "" internal $, so it is changed to "1", "1" + 1 will be converted to an integer, get 2, "1 ". 1 is converted to a string, which is "11"