PHP 5.2.x output operation details

$a = "hello";
$b = "world";
echo $a,$b,"";
echo $a.$b."< ;br>";
print $a.$b;

output result: helloworld helloworld helloworld

But this will cause a compilation error:

print $a,$b;

Note: There is a comma in the middle of $a $b. 2. The difference between single quotes and double quotes:

$a = "hello";
$b = "world";
echo "$a $b";

Output: hello world

echo '$a $b'; Then the output is: $a $b

The following can also be output normally:

$arr = array("a" => 1);
echo $arr['a'];