PHP If First Word Equals

Hi,

I was wondering what would be the most efficient way of finding and testing against the first word (including first white space) within a string.

So say i had a variable $a that equaled something like 'P001 example word'. If i wanted to test this true or false based on whether 'P001 ' was found and first word in the string, what would be the best way of doing so?

Any help would be appreciated.

Thanks.