Questions about but double quotes and plus and dot numbers

Questions about but double quotes and plus and dot numbers
I want to output $a = 1 on the screen, so I wrote the following, but only the last one is right.
There are two points to ask,
1th is the first two why only output the number, the front of the "$a =" and "$a =" is how to deal with it?
2nd, what exactly does the plus sign and dot number represent in PHP? Why didn't the two front signs have an error?
$a = 1;
echo "$a =" + $a; Error 2
echo ' $a = ' + $a; Error 1
echo "$a =". $a; Fatal error 1=1
echo ' $a = '. $a; Correct $a=1
?>

Just contact PHP, read the information, did not find the two points of knowledge, so come up for help, but also hope that you PHP talent. Thanks, man.

------Solution--------------------
Double quotes resolve variables, and single quotes do not: Number is a string connector, + is the plus
------Solution--------------------
+ Arithmetic Operators
. String connector

------Solution--------------------
PHP parses the string, "" will resolve the internal escape or $, "will only escape the internal \ ', and no other."


echo "$a =" + $a; It is because of the "" internal $, so was replaced by "1", "1" + 1 will turn into integers, get 2, "1". 1 is converted to a string, which is "11"

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