How to compare tables from different mySQL databases with PHP?

I’m new to PHP as well as the field and have been tasked with finding inconsistencies amongst tables from different mySQL databases. Every table should be the same between databases (column names, column count), with the exception of the actual data held in it, which they are not. I will need to identify the offending columns, the table they are in and, the database they are in. Below is my code thus far:

<?php
chdir(<path>);
include(<file>);
include(<file>);

$db = new db($hostname, $user, $pass, $commondatabase);

// Get baseline DB columns
//$db->select_db('<baseDB>');
//$btq = "SHOW TABLES";
//$btr = $db->query($btq);
//while($trows = $db->fetch_assoc($btr) {
//      $bcq = "SHOW COLUMNS FROM ".$btr[<key>];
//      $bcr = "$db->query($bcq);
//      $basecolumns[] = $bcr;
//}

$dbq = "SELECT * FROM <commonDB>.<DBnames> ORDER BY <DBnamesID>";
$dbr = $db->query($dbq);

while($dbrow = $db->fetch_assoc($dbr)) {
echo "n";
print_r($dbrow['dbname']);
echo "n";
        $db->select_db($dbrow['dbname']);
        $tq = "SHOW TABLES";
        $tr = $db->query($tq);
        while($table = $db->fetch_assoc($tr)) {
/*print_r($dbtables);*/
                $cq = "SHOW COLUMNS FROM ".$table['Tables_in_'.$dbrow['dbname']];
                $cr = $db->query($cq);
                $dbcols = [];
                while($col = $db->fetch_assoc($cr)) {
/*print_r($col);*/      $dbcols = $col;
                        // Do check against baseline here
                        //if($dbcols != $basecolumns) {
                        //      $badcolumns[] = $dbcols;
                        //}
                }
        }
}


$db->close();
?>

Currently it will loop down to the columns, but the output is not visually pleasing nor very manageable. My first concern is to get this working to where I can loop down to the columns and actually get them in their own arrays to be checked against the baseline and I’ve hit a wall, so any direction would be much appreciated. Currently each column is being assigned to its own array versus an array of all the column names for the database and table the loop is on. TIA!

Answer

Here is what I came up with. Not sure if it’s the most DRY way to go about it, but it works and achieves the results I was looking for. Thank you to Barmar for sending me down the right path.

<?php

chdir(<path>);
include(<file>);
include(<file>);

$db = new db($hostname,$user,$pass,$commondatabase);

$db->select_db(<database>);

// Get tables from baseline database
$q="select distinct <column-name> from <table-name> where <where-param> ORDER BY <order-by-param>";
$r=$db->query($q);
while($tables=$db->fetch_assoc($r)) {
        $table = $tables[<key>];

        $x = array();
        $x["$table"]=array();

        // Get table columns from baseline database
        $q="select <column-name> from <table-name> where <where-param> and <where-param> ORDER BY <order-by-param>";
        $r2=$db->query($q);
        while ($columns=$db->fetch_assoc($r2)) {
                $x["$table"][]=$columns[<key>];
        }

        // Check other databases for this table
        $q="select * from $commondatabase.<table-with-database-names> ";
        $q.="where <where-param> order by <order-by-param>";
        $r2=$db->query($q);
        while ($databases=$db->fetch_assoc($r2)) {

                $y = array();
                $y["$table"]=array();

                // Get table columns in $databases[<key>]
                $q="select <column-name> from <table-name> where <where-param> and <where-param> ORDER BY <order-by-param>";
                $r3=$db->query($q);
                while ($columns=$db->fetch_assoc($r3)) {
                        $y["$table"][]=$columns[<key>];
                }

                // Check against baseline
                $z1 = array_diff($x["$table"],$y["$table"]);
                $z2 = array_diff($y["$table"],$x["$table"]);

                // Echo out comparison results
                if (empty($z1) && empty($z2)) {
                        //echo "all good.n";
                } else {
                        echo "Difference found in {$databases[<key>]}.{$tables[<key>]}:";
                        if (!empty($z1)) print_r($z1);
                        if (!empty($z2)) print_r($z2);
                }
        }
}

$db->close();
?>