PHP checks the code for whether an extension library or function is available

In fact, the functions introduced in this article are already in the PHP manual, but because of the strong independence of these functions, not easy to find, so a separate introduction, easy to consult.
Get_loaded_extensions this function returns all the (available) modules that have been loaded.
Usage:

 
print_r(get_loaded_extensions()); 

Get_extension_funcs this function returns all available functions of the specified module. The parameters passed in (module name) must be lowercase
Usage:

 
print_r(get_extension_funcs("gd")); 

Get_defined_functions this function returns all defined functions, both built-in and user-defined.
Usage:

 
function myrow($id, $data){ 
return "<tr><th>$id</th><td>$data</td></tr>n"; 
} 
$arr = get_defined_functions(); 
print_r($arr); 

Output:

 
Array 
( 
[internal] => Array 
( 
[0] => zend_version 
[1] => func_num_args 
[2] => func_get_arg 
[3] => func_get_args 
[4] => strlen 
[5] => strcmp 
[6] => strncmp 
... 
[750] => bcscale 
[751] => bccomp 
) 
[user] => Array 
( 
[0] => myrow 
) 
) 

Where $arr["internal"] is a built-in function, and $arr["user"] is a user-defined function.
Checks whether the specified function exists -function_exists the function returns whether the specified function is defined.
Usage:

 
if (function_exists('imap_open')) { 
echo "IMAP functions are available.<br />n"; 
} else { 
echo "IMAP functions are not available.<br />n"; 
}