PHP - I'm Getting an Array to String Conversion Warning When Using Array_Merge

Question

I keep getting an "Array to String" conversion warning when using array_merge(). I don't know why. I've var_dump() the is_array function for $u, $g, $und_schools, and the results of the array_merge function and array_unique function and it always returns true for is_array.

I don't know why I keep getting this error.

Here's the code below. I commented on the lines where I get the error. Anyone have insights?

<?php
$undergrad_schools = array_filter($this->structure["schools"], function ($b) {
    foreach ($b as $a) {
        if (
            (in_array("bachelors", $a["degrees"]) ||
                in_array("certificate", $a["degrees"])) &&
            $a["info"]["state"] != "PR"
        ) {
            return true;
        } elseif (
            (in_array("masters", $a["degrees"]) ||
                in_array("doctorate", $a["degrees"])) &&
            $a["info"]["state"] != "PR"
        ) {
            return false;
        } else {
            return false;
        }
    }
});

$grad_schools = array_filter($this->structure["schools"], function ($b) {
    foreach ($b as $a) {
        if (
            (in_array("bachelors", $a["degrees"]) ||
                in_array("certificate", $a["degrees"])) &&
            $a["info"]["state"] != "PR"
        ) {
            return false;
        } elseif (
            (in_array("masters", $a["degrees"]) ||
                in_array("doctorate", $a["degrees"])) &&
            $a["info"]["state"] != "PR"
        ) {
            return true;
        } else {
            return false;
        }
    }
});

////// END IGNORE

//flatten the undergrad and grad schools arrays

$und_schools = [];
$grd_schools = [];

//Array to string conversion notice happens on line 37 and line 41 (or where array_merge occurs)

foreach ($undergrad_schools as $u) {
    $und_schools = array_merge(array_unique($und_schools), $u);
}

//Array to string conversion notice happens on line 37 and line 41 (or where array_merge occurs)

foreach ($grad_schools as $g) {
    $grd_schools = array_merge(array_unique($grd_schools), $g);
}