<?php
$array = array_fill(0, 10, array_fill(0, 10, '_'));
 
$array[0][0] = '1';
 
$i = 0;
 
 
foreach($array as $keyX => $x) {
 
	foreach($x as $keyY => $y) {
	//Желаеш изменить данный элент массива, тогда к нему и обращайся. По соответствующим координатам.
		$array[$keyX][$keyY] = $i;
 
		$i++;
	}
}
 
foreach ($array as $y => $x) {
	foreach ($x as $key => $value) {
		echo ' ' . $value . ' ';
	}
 
	echo "\n";
}

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