"text" + var + "more text"), in PHP I always used
"text" . $var . "more text". That may change soon, though, as it seems that embedding variables in string literals is a bit faster.
For example, a JPG cannot hold transparency; it must have a background color of some type. However, as a tradeoff, it is what is called a "lossy" format, which means that some data will be lost when it is saved due to being compressed.
PNG, however, is saved in a "lossless" format (as-is), which means the file you see as a PNG is eye-to-eye identical to what was originally saved (in many cases, a PSD file). PNGs can be slightly compressed, but not by a lot.