[php7] fix haxe.io.Input.readAll() with disabled analyzer optimizations (closes #6387)

std/php7/_std/haxe/io/BytesBuffer.hx

@@ -69,7 +69,7 @@ class BytesBuffer {
 		if( pos < 0 || len < 0 || pos + len > src.length ) {
 			throw Error.OutsideBounds;
 		} else {
-			Syntax.binop(b, '.=', src.getData().sub(pos, len).toString());
+			b += src.getData().sub(pos, len).toString();
 		}
 	}