How efficient is PHP in introducing functions in foreach?

For example, I run a foreach loop:

<code>foreach(func() as $row){
echo $row;
}
</code>

At this time, does func() run once or multiple times?
Do you need to use it? As mentioned on the Internet, in order to optimize efficiency, define it first:

<code>$arr=func();</code>

Execute again:

<code>foreach($arr as $row){
    echo $row;
}</code>