It means the function will work correctly when you pass it arbitrary binary data (i.e. strings containing non-ASCII bytes and/or null bytes).

For example, a non-binary-safe function might be based on a C function which expects null-terminated strings, so if the string contains a null character, the function would ignore anything after it.

This is relevant because PHP does not cleanly separate string and binary data.

For instance, if PHP’s strlen function worked like C standard library strlen, the result here would be wrong:

如果函数是binary safe的话，我们将得到7；如果函数是非binary safe的话，我们将得到3 * strlen是binary safe的，所以实际上以下的运行结果是"7"

$str = "abc\x00abc";
echo strlen($str); //gives 7, not 3!