$ expode_top []＆$ expode_bottom []用法

I'm rewriting a PHP plugin, and there are functions called $expode_top[] & $expode_bottom[]. I understand what the normal explode function does, but what are these?

It seems to be impossible to find an answer on Google because it replaces the underscore with a space.

Those are array variables, not functions, they just happen to start with a keyword you are familiar with. Anything beginning with a $ is a variable in PHP.

Using [] will put the assigned variable into the "next" position of the array. For example:

$expode_top = array();
$expode_top[] = "testing";

if ( $expode_top[0] == "testing" ){
    echo "it does equal testing";
}

As @gwillie rightly commented, it could also be a variable function - the name of the variable is replaced and then that function is executed. Second example:

$expode_top = "echo";
$expode_top("testing");

Is functionally the same as:

echo("testing");

Those are array variable names, not functions, as indicated by the $ and []. Also, neither exists as a function.