Adding a string into another string

[IFR]

I would like to add a string into another string
for example:

$str="abcdef";
$add= "00";
$newstr=$add + $str;

so $newstr should be "00abcdef"
of course it doesnt work this way

[paulnaj]

Concatenation operator is '.'

$a = "RT";
$b = "FM";
$message = $a.$b

echo $message;

[IFR]

i feel so dumb 🙁