$array = array('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');
$permutations = array();
$total = count($array);
for ($i = 0; $i < $total; $i++) {
for ($j = 0; $j < $total; $j++) {
$str = $array[$i] . $array[$j];
array_push($permutations, $str);
}
}
print_r($permutations);
/*
run:
Array
(
[0] => aa
[1] => ab
[2] => ac
[3] => ad
[4] => ae
[5] => af
[6] => ag
[7] => ah
[8] => ai
[9] => aj
[10] => ak
[11] => al
[12] => am
[13] => an
[14] => ao
[15] => ap
[16] => aq
[17] => ar
[18] => as
[19] => at
[20] => au
[21] => av
[22] => aw
[23] => ax
[24] => ay
[25] => az
[26] => ba
[27] => bb
[28] => bc
[29] => bd
[30] => be
[31] => bf
[32] => bg
[33] => bh
[34] => bi
[35] => bj
[36] => bk
[37] => bl
[38] => bm
[39] => bn
[40] => bo
[41] => bp
[42] => bq
[43] => br
[44] => bs
[45] => bt
[46] => bu
[47] => bv
[48] => bw
[49] => bx
[50] => by
[51] => bz
[52] => ca
[53] => cb
[54] => cc
[55] => cd
[56] => ce
[57] => cf
[58] => cg
[59] => ch
[60] => ci
[61] => cj
[62] => ck
[63] => cl
[64] => cm
[65] => cn
[66] => co
[67] => cp
[68] => cq
[69] => cr
[70] => cs
[71] => ct
[72] => cu
[73] => cv
[74] => cw
[75] => cx
[76] => cy
[77] => cz
[78] => da
[79] => db
[80] => dc
[81] => dd
[82] => de
[83] => df
[84] => dg
[85] => dh
[86] => di
[87] => dj
[88] => dk
[89] => dl
[90] => dm
[91] => dn
[92] => do
[93] => dp
[94] => dq
[95] => dr
[96] => ds
[97] => dt
[98] => du
[99] => dv
[100] => dw
[101] => dx
[102] => dy
[103] => dz
[104] => ea
[105] => eb
[106] => ec
[107] => ed
[108] => ee
[109] => ef
[110] => eg
[111] => eh
[112] => ei
[113] => ej
[114] => ek
[115] => el
[116] => em
[117] => en
[118] => eo
[119] => ep
[120] => eq
[121] => er
[122] => es
[123] => et
[124] => eu
[125] => ev
[126] => ew
[127] => ex
[128] => ey
[129] => ez
[130] => fa
[131] => fb
[132] => fc
[133] => fd
[134] => fe
[135] => ff
[136] => fg
[137] => fh
[138] => fi
[139] => fj
[140] => fk
[141] => fl
[142] => fm
[143] => fn
[144] => fo
[145] => fp
[146] => fq
[147] => fr
[148] => fs
[149] => ft
[150] => fu
[151] => fv
[152] => fw
[153] => fx
[154] => fy
[155] => fz
[156] => ga
[157] => gb
[158] => gc
[159] => gd
[160] => ge
[161] => gf
[162] => gg
[163] => gh
[164] => gi
[165] => gj
[166] => gk
[167] => gl
[168] => gm
[169] => gn
[170] => go
[171] => gp
[172] => gq
[173] => gr
[174] => gs
[175] => gt
[176] => gu
[177] => gv
[178] => gw
[179] => gx
[180] => gy
[181] => gz
[182] => ha
[183] => hb
[184] => hc
[185] => hd
[186] => he
[187] => hf
[188] => hg
[189] => hh
[190] => hi
[191] => hj
[192] => hk
[193] => hl
[194] => hm
[195] => hn
[196] => ho
[197] => hp
[198] => hq
[199] => hr
[200] => hs
[201] => ht
[202] => hu
[203] => hv
[204] => hw
[205] => hx
[206] => hy
[207] => hz
[208] => ia
[209] => ib
[210] => ic
[211] => id
[212] => ie
[213] => if
[214] => ig
[215] => ih
[216] => ii
[217] => ij
[218] => ik
[219] => il
[220] => im
[221] => in
[222] => io
[223] => ip
[224] => iq
[225] => ir
[226] => is
[227] => it
[228] => iu
[229] => iv
[230] => iw
[231] => ix
[232] => iy
[233] => iz
[234] => ja
[235] => jb
[236] => jc
[237] => jd
[238] => je
[239] => jf
[240] => jg
[241] => jh
[242] => ji
[243] => jj
[244] => jk
[245] => jl
[246] => jm
[247] => jn
[248] => jo
[249] => jp
[250] => jq
[251] => jr
[252] => js
[253] => jt
[254] => ju
[255] => jv
[256] => jw
[257] => jx
[258] => jy
[259] => jz
[260] => ka
[261] => kb
[262] => kc
[263] => kd
[264] => ke
[265] => kf
[266] => kg
[267] => kh
[268] => ki
[269] => kj
[270] => kk
[271] => kl
[272] => km
[273] => kn
[274] => ko
[275] => kp
[276] => kq
[277] => kr
[278] => ks
[279] => kt
[280] => ku
[281] => kv
[282] => kw
[283] => kx
[284] => ky
[285] => kz
[286] => la
[287] => lb
[288] => lc
[289] => ld
[290] => le
[291] => lf
[292] => lg
[293] => lh
[294] => li
[295] => lj
[296] => lk
[297] => ll
[298] => lm
[299] => ln
[300] => lo
[301] => lp
[302] => lq
[303] => lr
[304] => ls
[305] => lt
[306] => lu
[307] => lv
[308] => lw
[309] => lx
[310] => ly
[311] => lz
[312] => ma
[313] => mb
[314] => mc
[315] => md
[316] => me
[317] => mf
[318] => mg
[319] => mh
[320] => mi
[321] => mj
[322] => mk
[323] => ml
[324] => mm
[325] => mn
[326] => mo
[327] => mp
[328] => mq
[329] => mr
[330] => ms
[331] => mt
[332] => mu
[333] => mv
[334] => mw
[335] => mx
[336] => my
[337] => mz
[338] => na
[339] => nb
[340] => nc
[341] => nd
[342] => ne
[343] => nf
[344] => ng
[345] => nh
[346] => ni
[347] => nj
[348] => nk
[349] => nl
[350] => nm
[351] => nn
[352] => no
[353] => np
[354] => nq
[355] => nr
[356] => ns
[357] => nt
[358] => nu
[359] => nv
[360] => nw
[361] => nx
[362] => ny
[363] => nz
[364] => oa
[365] => ob
[366] => oc
[367] => od
[368] => oe
[369] => of
[370] => og
[371] => oh
[372] => oi
[373] => oj
[374] => ok
[375] => ol
[376] => om
[377] => on
[378] => oo
[379] => op
[380] => oq
[381] => or
[382] => os
[383] => ot
[384] => ou
[385] => ov
[386] => ow
[387] => ox
[388] => oy
[389] => oz
[390] => pa
[391] => pb
[392] => pc
[393] => pd
[394] => pe
[395] => pf
[396] => pg
[397] => ph
[398] => pi
[399] => pj
[400] => pk
[401] => pl
[402] => pm
[403] => pn
[404] => po
[405] => pp
[406] => pq
[407] => pr
[408] => ps
[409] => pt
[410] => pu
[411] => pv
[412] => pw
[413] => px
[414] => py
[415] => pz
[416] => qa
[417] => qb
[418] => qc
[419] => qd
[420] => qe
[421] => qf
[422] => qg
[423] => qh
[424] => qi
[425] => qj
[426] => qk
[427] => ql
[428] => qm
[429] => qn
[430] => qo
[431] => qp
[432] => qq
[433] => qr
[434] => qs
[435] => qt
[436] => qu
[437] => qv
[438] => qw
[439] => qx
[440] => qy
[441] => qz
[442] => ra
[443] => rb
[444] => rc
[445] => rd
[446] => re
[447] => rf
[448] => rg
[449] => rh
[450] => ri
[451] => rj
[452] => rk
[453] => rl
[454] => rm
[455] => rn
[456] => ro
[457] => rp
[458] => rq
[459] => rr
[460] => rs
[461] => rt
[462] => ru
[463] => rv
[464] => rw
[465] => rx
[466] => ry
[467] => rz
[468] => sa
[469] => sb
[470] => sc
[471] => sd
[472] => se
[473] => sf
[474] => sg
[475] => sh
[476] => si
[477] => sj
[478] => sk
[479] => sl
[480] => sm
[481] => sn
[482] => so
[483] => sp
[484] => sq
[485] => sr
[486] => ss
[487] => st
[488] => su
[489] => sv
[490] => sw
[491] => sx
[492] => sy
[493] => sz
[494] => ta
[495] => tb
[496] => tc
[497] => td
[498] => te
[499] => tf
[500] => tg
[501] => th
[502] => ti
[503] => tj
[504] => tk
[505] => tl
[506] => tm
[507] => tn
[508] => to
[509] => tp
[510] => tq
[511] => tr
[512] => ts
[513] => tt
[514] => tu
[515] => tv
[516] => tw
[517] => tx
[518] => ty
[519] => tz
[520] => ua
[521] => ub
[522] => uc
[523] => ud
[524] => ue
[525] => uf
[526] => ug
[527] => uh
[528] => ui
[529] => uj
[530] => uk
[531] => ul
[532] => um
[533] => un
[534] => uo
[535] => up
[536] => uq
[537] => ur
[538] => us
[539] => ut
[540] => uu
[541] => uv
[542] => uw
[543] => ux
[544] => uy
[545] => uz
[546] => va
[547] => vb
[548] => vc
[549] => vd
[550] => ve
[551] => vf
[552] => vg
[553] => vh
[554] => vi
[555] => vj
[556] => vk
[557] => vl
[558] => vm
[559] => vn
[560] => vo
[561] => vp
[562] => vq
[563] => vr
[564] => vs
[565] => vt
[566] => vu
[567] => vv
[568] => vw
[569] => vx
[570] => vy
[571] => vz
[572] => wa
[573] => wb
[574] => wc
[575] => wd
[576] => we
[577] => wf
[578] => wg
[579] => wh
[580] => wi
[581] => wj
[582] => wk
[583] => wl
[584] => wm
[585] => wn
[586] => wo
[587] => wp
[588] => wq
[589] => wr
[590] => ws
[591] => wt
[592] => wu
[593] => wv
[594] => ww
[595] => wx
[596] => wy
[597] => wz
[598] => xa
[599] => xb
[600] => xc
[601] => xd
[602] => xe
[603] => xf
[604] => xg
[605] => xh
[606] => xi
[607] => xj
[608] => xk
[609] => xl
[610] => xm
[611] => xn
[612] => xo
[613] => xp
[614] => xq
[615] => xr
[616] => xs
[617] => xt
[618] => xu
[619] => xv
[620] => xw
[621] => xx
[622] => xy
[623] => xz
[624] => ya
[625] => yb
[626] => yc
[627] => yd
[628] => ye
[629] => yf
[630] => yg
[631] => yh
[632] => yi
[633] => yj
[634] => yk
[635] => yl
[636] => ym
[637] => yn
[638] => yo
[639] => yp
[640] => yq
[641] => yr
[642] => ys
[643] => yt
[644] => yu
[645] => yv
[646] => yw
[647] => yx
[648] => yy
[649] => yz
[650] => za
[651] => zb
[652] => zc
[653] => zd
[654] => ze
[655] => zf
[656] => zg
[657] => zh
[658] => zi
[659] => zj
[660] => zk
[661] => zl
[662] => zm
[663] => zn
[664] => zo
[665] => zp
[666] => zq
[667] => zr
[668] => zs
[669] => zt
[670] => zu
[671] => zv
[672] => zw
[673] => zx
[674] => zy
[675] => zz
)
*/
