if(function_exists('my_function')){
    // my_function is defined
}

<?php
  if (!function_exists('myfunction')) {

     function myfunction()
     {
       //write function statements
     }

}
 ?>

print_r(get_defined_functions());

<?php
// This will print "foo defined"
if (function_exists('foo')) {
  print "foo defined";
} else {
  print "foo not defined";
}
//note even though the function is defined here, it previously was told to have already existed
function foo() {}

<?php
// This will print "defining bar" and will define the function bar
if (function_exists('bar')) {
  print "bar defined";
} else {
  print "defining bar";
  function bar() {}
}

$arrFun = array('fun1','fun2','fun3');
if(is_array($arrFun)){
  $arrMsg = array();
  foreach ($arrFun as $key => $value) {
    if(!function_exists($value)){
      $arrMsg[] = $value;
    }
  }
  foreach ($arrMsg as $key => $value) {
    echo "{$value} function is does not exist <br/>";
  }
}
function fun1(){
}

Output

fun2 function is does not exist
fun3 function is does not exist