This sounds like a logic question, not so much a PHP-specific question.
Here's the logic you need to examine:
A. Loop through the articles you want to display
B. Compare the date of the present article with the date of the previous article
C. If the date is different or no previous date has been stored, insert a line break (you could split this up into two if statements if you dont want a line break on "no date has been stored")
D. Display the article
E. Remember the date of this article in a variable for comparison on the next article
Pseudo Code:
if(!$rememberedArticleDate || $rememberedArticleDate!=$currentArticleDate) {
// ... insert the break (html stuffs)
}
// display article (html stuffs)
$rememberedArticleDate = $currentArticleDate;