This is growing in the woods near my house in the Butler, Pennsylvania area. I have not seen it around before and I have no idea what is it. What is it? Is it poisonous to pets (or little kids)?
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That looks like a bit like a Smilax to me, possibly Smilax herbacea.
Make sure you have a positive ID on any wild plant before you eat it, but if this is Smilax it looks like you don't have to worry about it being poisonous: from Flora of North America entry on the genus Smilax:
Smilax has numerous uses. Sarsaparilla, a beverage and medicinal used against rheumatism, is obtained from the rhizomes of various species... All species of Smilax are excellent wildlife food and are also browsed, or the rhizomes dug and eaten, by domestic stock.
For you, or for other Pennsylvania people, note that the Pennsylvania Flora Project of the Morris Aboretum is a great resource. You can download species lists by county, and just that and some google image searches can often substitute for a field-guide of your county.
If you want to identify plants the hard way, and are willing to deal with special botanical vocabulary, the Flora of North America that I linked to above is the definitive resource.