[PHP]利用openssl_random_pseudo_bytes和base64_encode函数来生成随机字符串
openssl_random_pseudo_bytes函数本身是用来生成指定个数的随机字节，因此在使用它来生成随机字符串时，还需要配合使用函数base64_encode。如下所示：
<code class="language-php hljs "><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-function"><span class="hljs-keyword">function</span> <span class="hljs-title">getRandomString</span><span class="hljs-params">(<span class="hljs-variable">$length</span> = <span class="hljs-number">42</span>)</span> {</span> <span class="hljs-comment">/* * Use OpenSSL (if available) */</span> <span class="hljs-keyword">if</span> (function_exists(<span class="hljs-string">'openssl_random_pseudo_bytes'</span>)) { <span class="hljs-variable">$bytes</span> = openssl_random_pseudo_bytes(<span class="hljs-variable">$length</span> * <span class="hljs-number">2</span>); <span class="hljs-keyword">if</span> (<span class="hljs-variable">$bytes</span> === <span class="hljs-keyword">false</span>) <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException(<span class="hljs-string">'Unable to generate a random string'</span>); <span class="hljs-keyword">return</span> substr(str_replace([<span class="hljs-string">'/'</span>, <span class="hljs-string">'+'</span>, <span class="hljs-string">'='</span>], <span class="hljs-string">''</span>, base64_encode(<span class="hljs-variable">$bytes</span>)), <span class="hljs-number">0</span>, <span class="hljs-variable">$length</span>); } <span class="hljs-variable">$pool</span> = <span class="hljs-string">'0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'</span>; <span class="hljs-keyword">return</span> substr(str_shuffle(str_repeat(<span class="hljs-variable">$pool</span>, <span class="hljs-number">5</span>)), <span class="hljs-number">0</span>, <span class="hljs-variable">$length</span>); }</code>在调用base64_encode函数之后，还对结果进行了一次替换操作，目的是要去除随机生成的字符串中不需要的字符。
当然，在使用openssl_random_pseudo_bytes函数之前，最好使用function_exists来确保该函数在运行时是可用的。如果不可用，则使用Plan B：
<code class="language-php hljs ">substr(str_shuffle(str_repeat(<span class="hljs-variable">$pool</span>, <span class="hljs-number">5</span>)), <span class="hljs-number">0</span>, <span class="hljs-variable">$length</span>);</code>
这个函数的通用性很强，可以根据业务的需要进行适当修改然后当作静态方法进行调用。
