<?php
 
mb_internal_encoding('utf-8');
 
$text = "Аргентина манит негра";
$result= "Палиндром";
$result2= "Не палиндром";
$text = mb_strtolower($text);
 
$text = str_replace(" ","",$text);
 
$length = mb_strlen($text);
$halfLength = floor($length/2);
for ($i = 0; $i <= $halfLength;) {
	$o=-$i -1;
	$comparison1=mb_substr($text,$i,1);
    $comparison2=mb_substr($text,$o,1);
	 if ($comparison1==$comparison2) {
	 	$i++;
	 } else { 
	 	echo "Результат $result2";
	 	break;
	 }
}
if ($i == $halfLength + 1) {
 	echo "Результат: $result";
}

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