I saw this string from an ajax call on some. It's clearly some sort of template. a:15 means there're 15 items in the {} expression. i:0 means item0, s:63: means the length of the string after it. I google for a while, but could not find any JS template engine that can take input like this one. It is possible they use Regex to parse the data.
a:15:{i:0;s:63:\"http://ww2.some.web/mw600/c01b8166jw1e4cf9fu2s0j20dw08v0v4.jpg\";i:1;s:63:\"http://ww4.some.web/mw600/c01b8166jw1e4cf9h284bj20dw0980ut.jpg\";i:2;s:63:\"http://ww1.some.web/mw600/c01b8166jw1e4cf9ksczrj20dw097n20.jpg\";i:3;s:63:\"http://ww3.some.web/mw600/c01b8166jw1e4cf9jvzymj20dw09840f.jpg\";i:4;s:63:\"http://ww2.some.web/mw600/c01b8166jw1e4cf9m9j9rj20dw0av41i.jpg\";i:5;s:63:\"http://ww3.some.web/mw600/c01b8166jw1e4cf9n1iq2j20dw0990ue.jpg\";i:6;s:63:\"http://ww2.some.web/mw600/c01b8166jw1e4cf9q062tj20dw09en17.jpg\";i:7;s:63:\"http://ww3.some.web/mw600/c01b8166jw1e4cf9sprglj20dw0a1djh.jpg\";i:8;s:63:\"http://ww1.some.web/mw600/c01b8166jw1e4cf9srts5j20dw097jui.jpg\";i:9;s:63:\"http://ww2.some.web/mw600/c01b8166jw1e4cf9wj84oj20dw08zn02.jpg\";i:10;s:63:\"http://ww1.some.web/mw600/c01b8166jw1e4cf9ws795j20dw09o418.jpg\";i:11;s:63:\"http://ww3.some.web/mw600/c01b8166jw1e4cf9xpixhj20dw0990ty.jpg\";i:12;s:63:\"http://ww3.some.web/mw600/c01b8166jw1e4cfa05o8fj20dw099die.jpg\";i:13;s:63:\"http://ww4.some.web/mw600/c01b8166jw1e4cfa0ah9yj20dw0aa76h.jpg\";i:14;s:63:\"http://ww3.some.web/mw600/c01b8166jw1ehttp://ww2.some.web/mw600/c01b8166jw1e4cf9fu2s0j20dw08v0v4.jpgcfa1jpsaj20dw099myq.jpg\";}
Looks like result of PHP serialize() function You can use this js function to parse it.