function_exists（）是否缓存其查询？

I wonder to know does function_exists() cache its queries internally?

No, it does not. It just checks whether the function is defined in the function table. Simple enough.

However, the Zend Opcache extension may optimize out some calls to function_exists() and certain other functions which can in some cases be evaluated at compile time. (It only optimizes out calls to function_exists() where the function is defined internally by PHP, or by an extension.)

No it doesn't, the following performance tests are confirming the fact:

function function_exists_cached($function_name)
{
    static $cache = array();
    if(isset($cache[$function_name]))
        return $cache[$function_name];
    return ($cache[$function_name] = \function_exists($function_name));

}
$funcs = \array_shift(\get_defined_functions());
$a = new \core\utiles\loadTime;
$times = $tot = 0;
$a->start();
/**
 * None Cached: No cache used here
 **/
for($index = 0; $index<count($funcs); $index++)
{
    foreach($funcs as $func)
    {
        $times++;
        if(\function_exists($func)) ;
    }
}
$s = $a->stop();
$tot+=$s;
echo "<b>None Cached : </b><b><span style='color:green'>#$times</span></b> function check took : $s seconds<br />";
$b = new \core\utiles\loadTime;
$times = 0;
$b->start(); 
/**
 * Fly Cached: An inline cache used here
 **/
static $func_check = array();
for($index = 0; $index<count($funcs); $index++)
{
    foreach($funcs as $func)
    {
        $times++;
        if(!isset($func_check[$func]))
        {
            if(\function_exists($func)) ;
            $func_check[$func] = 1;
        }
        else $func_check[$func];
    }
}
$s = $b->stop();
$tot+=$s;
echo "<b>Fly Cached : </b><b><span style='color:green'>#$times</span></b> function check took : $s seconds<br />";
$c = new \core\utiles\loadTime;
$times = 0;
$c->start();
/**
 * Function Cached: Has the same logic like Fly Cached section, 
 * But implemented in a function named function_exists_cached()
 **/
for($index = 0; $index<count($funcs); $index++)
{
    foreach($funcs as $func)
    {
        $times++;
        if(\function_exists_cached($func)) ;
    }
}
$s = $c->stop();
$tot+=$s;
echo "<b>Function Cached : </b><b><span style='color:green'>#$times</span></b> function check took : $s seconds<br />";
echo "Total time : $tot seconds";

outputs:

None Cached : #2365444 function check took : 0.69758 seconds
Fly Cached : #2365444 function check took : 0.5307 seconds
Function Cached : #2365444 function check took : 1.02575 seconds
Total time : 2.25403 seconds