This is part of the portulaca genus (sometimes spelled portulacca), closely related to the beautiful spiky leaved portulaca I used to keep in my front flowerbox, also known as Moss-Rose, which is a weedy ornamental flower. Moss-Rose will spread from where it was planted, sprawling around in sidewalk-cracks and such, but will not likely persist over many generations and spread to invasive proportions. Portulaca oleracea, on the other hand, is a very common weed here, growing low to the ground and creeping through other plants or appearing in sidewalk cracks. The seedlings appear like magic. If you put out a pot of dirt in South Philly, Purslane will probably appear in it before you know it as long as you keep pulling anything else that might keep it crowded out. Purslane is edible, and is one of the more nutritious plants one can eat, full of omega-3 fatty acids. Every year, I leave a few pots of dirt unplanted and just weed out anything that isn't edible and I end up with more Purslane than we can eat in the season.

Something that I have never read of, but experienced firsthand, is that purslane folds up in the night, standing erect as if to keep its leaves away from the dirt and protected from pests, molds and such. The picture below is the same patch as that in the above picture, taken around four in the morning.