求一个等式可填入的符号 [php]

<?php

// 计算 8 () 3 () 4=15 () 3 () 15 括号填写的符号且不重复

$a = 8;

$b = 3;

$c = 4;

$d = 15;

$dd = array('+','-','*','/');

while(true)

{

	$tt = []; // 空数组

	$t1 = $dd[rand(0,3)]; 

	$t2 = $dd[rand(0,3)];

	$t3 = $dd[rand(0,3)];

	$t4 = $dd[rand(0,3)];

	array_push($tt,$t1,$t2,$t3,$t4);

	// 判断是否有重复值

	if (count(array_unique($tt)) != count($tt))

	{

		continue; // 继续循环

	}

	// 左边

	$left = "8{$t1}3{$t2}4;";

	$left2 = eval("return ".$left);

	// 右边

	$right = "15{$t3}3{$t4}15;";

	$right2 = eval("return ".$right);

	if ($left2 == $right2)

	{

		echo $left2;

		echo '<br/>';

		echo $right2;

		echo '<br/>';

		echo trim($left,';').'='.trim($right,';');

		break;

	}

}