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Anyway, skip the whole switch lookup. (削除) chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up. (削除ここまで) See Rene Geuze's comment below; while chr works just fine, range is indeed more readable.

Anyway, skip the whole switch lookup. (削除) chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up. (削除ここまで) See Rene Geuze's comment below; while chr works just fine, range is indeed more readable.

Anyway, skip the whole switch lookup. chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up.

function get_rand($min, $max) {
 mt_srand((double) microtime() * 1000000);
 return mt_rand($min, $max);
}
function get_rand_alphanumeric($length) {
 $alnum = "";
 while(strlen($alnum) < $length) {
 $rand = get_rand(0, 35);
 if( $rand < 10 ) {
 $alnum .= (string) $rand;
 } else {
 $alnum .= chr($rand + 87); // lower-case
 }
 }
 return $alnum;
}
function get_rand_numbers($length) {
 if( $length <= 8 ) { // avoid int overflow
 return (string) get_rand(pow(10, $length-1), pow(10, $length));
 } else {
 $numbers = "";
 while(strlen($numbers) < $length) $numbers .= get_rand_numbers(8);
 return substr($numbers, 0, $length);
 }
}
function get_rand_letters($length) {
 $letters = "";
 while(strlen($letters) < $length) $letters .= chr(get_rand(97, 122));
 return $letters;
}

Also, the calls to mt_srand are by far the most expensive operation. Consider calling it less frequently, if speed is a concern. If I omit it, my functions are 7.5-10x faster; if I leave it as you see in the code, my functions are still faster, but only very marginally so (1.1x to 1.4x).

Anyway, skip the whole switch lookup. (削除) chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up. (削除ここまで) See Rene Geuze's comment below ; while chr works just fine, range is indeed more readable.

function get_rand($min, $max) {
 mt_srand((double) microtime() * 1000000);
 return mt_rand($min, $max);
}
function get_rand_alphanumeric($length) {
 $alnum = "";
 $range = range("a", "z");
 $limit = count($range) + 9;
 while(strlen($alnum) < $length) {
 $rand = get_rand(0, $limit);
 if( $rand < 10 ) {
 $alnum .= $rand;
 } else {
 $alnum .= $range[$rand - 10];
 }
 }
 return $alnum;
}
function get_rand_numbers($length) {
 if( $length <= 8 ) { // avoid int overflow
 return (string) get_rand(pow(10, $length-1), pow(10, $length));
 } else {
 $numbers = "";
 while(strlen($numbers) < $length) $numbers .= get_rand_numbers(8);
 return substr($numbers, 0, $length);
 }
}
function get_rand_letters($length) {
 $letters = "";
 $range = range("a", "z");
 $limit = count($range) - 1;
 while(strlen($letters) < $length) $letters .= $range[get_rand(0, $limit)];
 return $letters;
}

Also, the calls to mt_srand are by far the most expensive operation. Consider calling it less frequently, if speed is a concern. If I omit it, the functions above are 7.5-10x faster than the original; if I leave it as you see in the code, the functions are still faster, but only very marginally so (1.1x to 1.4x). In either case, get_rand_numbers obviously sees the biggest speedup since there's not lookup going on.

Edit 2 Replaced chr usage with a range array, re: the comments

First: I'm sure there's even simpler way than this (i.e. a library that just does what you need, period), but heck, I figured I'd try. Been years since I spent my days with PHP (don't particularly miss it).

Anyway, skip the whole switch lookup. chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up.

function get_rand($min, $max) {
 mt_srand((double) microtime() * 1000000);
 return mt_rand($min, $max);
}
function get_rand_alphanumeric($length) {
 $alnum = "";
 while(strlen($alnum) < $length) {
 if( get_rand(0, 1) ) {
 $alnum .= (string) get_rand(0, 9);
 } else {
 $alnum .= chr(get_rand(97, 122)); // lower-case
 }
 }
 return $alnum;
}
function get_rand_numbers($length) {
 if( $length <= 8 ) { // avoid int overflow
 return (string) get_rand(pow(10, $length-1), pow(10, $length));
 } else {
 $numbers = "";
 while(strlen($numbers) < $length) $numbers .= get_rand_numbers(8);
 return substr($numbers, 0, $length);
 }
}
function get_rand_letters($length) {
 $letters = "";
 while(strlen($letters) < $length) $letters .= chr(get_rand(97, 122));
 return $letters;
}

You get results like this

get_rand_alphanumeric(42) => 006njilgucr06gx2dtm4o9jp02v3me432ry043j2jq
get_rand_numbers(42) => 365818982423371436493339856184748778003731
get_rand_letters(42) => abfmthyuxdlganhfthebfjaugeeoniqawocgavowpx

First: I'm sure there's even simpler way than this (i.e. a library that just does what you need, period), but heck, I figured I'd try. Been years since I spent my days with PHP (don't particularly miss it).

Anyway, skip the whole switch lookup. chr() will take an int and give you its ASCII value. The 97-122 range (inclusive) is a-z (65-90 is A-Z). As for numbers, well, that's what a random function gives you - no need to look that up.

function get_rand($min, $max) {
 mt_srand((double) microtime() * 1000000);
 return mt_rand($min, $max);
}
function get_rand_alphanumeric($length) {
 $alnum = "";
 while(strlen($alnum) < $length) {
 $rand = get_rand(0, 35);
  if( $rand < 10 ) {
 $alnum .= (string) $rand;
 } else {
 $alnum .= chr($rand + 87); // lower-case
 }
 }
 return $alnum;
}
function get_rand_numbers($length) {
 if( $length <= 8 ) { // avoid int overflow
 return (string) get_rand(pow(10, $length-1), pow(10, $length));
 } else {
 $numbers = "";
 while(strlen($numbers) < $length) $numbers .= get_rand_numbers(8);
 return substr($numbers, 0, $length);
 }
}
function get_rand_letters($length) {
 $letters = "";
 while(strlen($letters) < $length) $letters .= chr(get_rand(97, 122));
 return $letters;
}

You get results like this

get_rand_alphanumeric(42) => upop8eoome0y0av2qav1j7yn5linyjshiurc8lbjja
get_rand_numbers(42) => 365818982423371436493339856184748778003731
get_rand_letters(42) => abfmthyuxdlganhfthebfjaugeeoniqawocgavowpx

Edit I just realized that your original alphanumeric function will pick randomly from your entire list, meaning it's 2.6 times more likely that the pick will be a letter. I edited mine to do the same to keep the results similar.

Also, the calls to mt_srand are by far the most expensive operation. Consider calling it less frequently, if speed is a concern. If I omit it, my functions are 7.5-10x faster; if I leave it as you see in the code, my functions are still faster, but only very marginally so (1.1x to 1.4x).