<?php

$chars = 'abcd';
$out = array();
for($i=0;$i<100;$i++) for($j=0;$j<5;$j++) $out[$i] .= $chars{mt_rand(0, 3)};
print_r($out);
?>

Array
(
    [0] => bcbca
    [1] => bbadb
    [2] => cadbb
    [3] => dddad
    [4] => cadaa
    [5] => abdda
    [6] => adcad
    [7] => dabaa
    [8] => ccabb
    [9] => dcbbd
    [10] => ccdbc
    [11] => bcbad
    [12] => bcbab
    [13] => cbacb
    [14] => cdaab
    [15] => ccdda
    [16] => adcbc
    [17] => accab
    [18] => abadb
    [19] => daaac
    [20] => cbbba
    [21] => bbdcb
    [22] => dcbac
    [23] => bcdbb
    [24] => bacba
    [25] => ccbbd
    [26] => dddab
    [27] => dbdba
    [28] => daddd
    [29] => acbdb
    [30] => acada
    [31] => cdaca
    [32] => cdada
    [33] => baccd
    [34] => bdddc
    [35] => addba
    [36] => accab
    [37] => bdccd
    [38] => cdcab
    [39] => dcada
    [40] => cbcda
    [41] => cbabd
    [42] => caacb
    [43] => bbdcc
    [44] => cccdb
    [45] => cbcda
    [46] => cadac
    [47] => aaaba
    [48] => bcaab
    [49] => dbbac
    [50] => cdacb
    [51] => cbabd
    [52] => bdcdd
    [53] => acbad
    [54] => cdcbb
    [55] => ddbbc
    [56] => acadc
    [57] => adbbc
    [58] => adaaa
    [59] => daabd
    [60] => bccdc
    [61] => bcabb
    [62] => abcac
    [63] => aaccb
    [64] => aadcc
    [65] => baaaa
    [66] => daada
    [67] => dcdbd
    [68] => dabda
    [69] => dbdbb
    [70] => bbccd
    [71] => baadc
    [72] => dabdc
    [73] => cabdb
    [74] => dbcdb
    [75] => ccddb
    [76] => bdccc
    [77] => cbbad
    [78] => ccacb
    [79] => caddb
    [80] => dbcdc
    [81] => bddba
    [82] => adcdc
    [83] => dbaab
    [84] => adacd
    [85] => bbbcd
    [86] => bbcbd
    [87] => ddcab
    [88] => badab
    [89] => aaacd
    [90] => dcdba
    [91] => aaccc
    [92] => acadc
    [93] => caaab
    [94] => acccc
    [95] => bddda
    [96] => aaaba
    [97] => cdddb
    [98] => ccdcc
    [99] => acabc
)