I've got a client using the following meta tags related to keywords with the syntax as shown.

<meta name="keywords" content="keywords keyword syntax is not comma separated">
<meta name="keyword phrases" content="keyword phrase, another keyword phrase, syntax is phrases, separated by commas">

My assumptions are the following:

Meta "keyword phrases" is not only invalid HTML (name attribute must not include spaces) but seems to be non-existent as a means to optimize.
How do you think search engines handle that line of code?

Also, I've always used the syntax shown in the content attribute of the "keyword phrases" tag above as the syntax of the keywords meta tag like so:
<meta name="keywords" content="keyword phrase, another keyword phrase, single, keywords, following">

Is that the best option for the keywords meta tag? I've seen it without commas. How do search engines tend to treat both syntaxes?
Also, am I correct in saying that spiders put more weight on the keywords/phrases that appear first in the content attribute?

Thanks much,

My guess is that the SEs would treat "keyword phrases" pretty much the same as they treat the "keywords" META... that is, they'll basically ignore it.

Most SEs don't even seem to bother indexing the keywords META at all. Among the few who even index it to start with, none give it much importance. Format it however you prefer; it will make no discernable difference.