変数への文字列の追加は｢変数 .= "文字列"｣または｢変数 = 変数 . "文字列"｣

JavaScriptにあった変数への文字列追加の+はPHPでは使えない。次のように.を使う。

×ダメ
$b = "Hello! ";
$b += "World! ";

×ダメ
$b = "Hello! ";
$b += $b + "World! ";

○(変数$bは"Hello! World"になる)
$b = "Hello! ";
$b .= "World";

○(変数$bは"Hello! World"になる)
$b = "Hello! ";
$b = $b . "World";

source

<?php
$a = "Hello! ";
$a = $a + "World";
echo $a,"<br><br>";

$b = "Hello! ";
$b += "World! ";
echo $b,"<br><br>";

$c = "Hello! ";
$c .= "World";
echo $c,"<br><br>";

$d = "Hello! ";
$d = $d . "World";
echo $d,"<br><br>";
?>