Why this:

for ( $j = "a"; $j <= "z"; $j++ ){
 echo $j.", ";
} 

is outputing this:

a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z, aa, ab, ac, ad, ae, af, ag, ah, ai, aj, ak, al, am, an, ao, ap, aq, ar, as, at, au, av, aw, ax, ay, az, ba, bb, bc, bd, be, bf, bg, bh, bi, bj, bk, bl, bm, bn, ........ (etc. to the end) yw, yx, yy, yz, 

but if you change "z" letter for "y" all works great.