Quick and dirty, but mostly convenient, Apache Solr password hash generator in PHP (5.4+).

solrPassHasher.php

<?php

/**
 * Function to generate a salted password hash to be used in a 'security.json'
 * file to secure Apache Solr.
 * This is the core function that you can extract for use in your own code.
 *
 * @see https://lucene.apache.org/solr/guide/basic-authentication-plugin.html#enable-basic-authentication
 *
 * @param string $pw The password to hash
 *
 * @return array [$hashedPass, $encodedSalt]
 */
function generateSolrPassHash($pw) {
    $method = 'sha256';
    $salt = openssl_random_pseudo_bytes(32);
    $hashedPass = openssl_digest($salt . $pw, $method, true);
    $hashedPass = base64_encode(openssl_digest($hashedPass, $method, true));
    $encodedSalt = base64_encode($salt);

    return [$hashedPass, $encodedSalt];
}

/**
 * Helper to generate JSON output.
 *
 * @param string $user The username
 * @param string $pass Hashed password
 * @param string $salt Hashed salt
 *
 * @return string
 */
function generateJson($user, $pass, $salt) {
    $output = <<<'JSON'
{
    "authentication":{
        "blockUnknown": true,
        "class":"solr.BasicAuthPlugin",
        "credentials":{"[user]":"[password]"}
    },
    "authorization":{
        "class":"solr.RuleBasedAuthorizationPlugin",
        "permissions":[
            {
                "name":"security-edit",
                "role":"admin"
            }
        ],
        "user-role":{"[user]":"admin"}
    }
}
JSON;

    return str_replace(
        ['[user]', '[password]'],
        [$user, "$pass $salt"],
        $output
    );
}

/**
 * Helper function to check the PHP version running on this system.
 *
 * @param  string $scriptName
 */
function versionCheck($scriptName) {
    // PHP_VERSION_ID is available as of PHP 5.2.7, if our version is lower than
    // that, then emulate it.
    if (!defined('PHP_VERSION_ID')) {
        $version = explode('.', PHP_VERSION);

        define('PHP_VERSION_ID', ($version[0] * 10000 + $version[1] * 100 + $version[2]));
    }

    if (PHP_VERSION_ID < 50400) {
        printf(PHP_EOL . "\033[31m" . 'You need at least PHP version 5.4 to use %s!' . "\033[0m" . PHP_EOL . PHP_EOL, $scriptName);
        die();
    }
}

/**
 * Helper to cleanup argv for easy usage.
 *
 * @param array $argv
 *
 * @return array
 */
function processArgv($argv) {
    return array_values(array_filter($argv, function($val) {
        return stripos($val, '-') !== 0;
    }));
}

/**
 * Main program logic.
 */
function main($argv) {
    $argv = processArgv($argv);

    echo 'Quick and dirty, but mostly convenient, Apache Solr password hash generator in PHP.' . PHP_EOL;

    versionCheck($argv[0]);

    if (count($argv) < 2) {
        echo "Usage:" . PHP_EOL;
        printf(PHP_EOL . "\033[31m" . 'php %s "password to encode"' . "\033[0m" . PHP_EOL . PHP_EOL, $argv[0]);
        echo "  or to generate the full JSON file:" . PHP_EOL;
        printf(PHP_EOL . "\033[31m" . 'php %s -j "password to encode" "user"' . "\033[0m" . PHP_EOL . PHP_EOL, $argv[0]);
        die();
    }

    list($pass, $salt) = generateSolrPassHash($argv[1]);

    $output = sprintf(
        PHP_EOL . "Your password ('%s') in hashed form to be placed in the 'security.json' file is:" . PHP_EOL,
        $argv[1]
    );
    $output .= "    \033[32m$pass $salt\033[0m";

    $options = getopt('j', ['json']);
    if (
        isset($argv[2]) && (
            array_key_exists('j', $options) ||
            array_key_exists('json', $options)
        )
    ) {
        $output = PHP_EOL . "The contents of your 'security.json' file is:" . PHP_EOL;
        $output .= "\033[32m" . generateJson($argv[2], $pass, $salt) . "\033[0m";
    }

    echo $output . PHP_EOL . PHP_EOL;
}

main($argv);