php中var_dump()函数的详解说明

作者：袖梨 2022-06-24

var_dump()

void var_dump ( mixed expression [, mixed expression [, ...]] )

var_dump()方法是判断一个变量的类型与长度,并输出变量的数值,如果变量有值输的是变量的值并回返数据类型.
此函数显示关于一个或多个表达式的结构信息，包括表达式的类型与值。数组将递归展开值，通过缩进显示其结构。

例1

代码如下	复制代码
<?php $a = "alsdflasdf;a"; $b = var_dump($a); echo " "; //var_dump($c); $d=var_dump($c); echo " "; echo $a; echo " "; echo $b; echo " "; 输出： string(12) "alsdflasdf;a" NULL alsdflasdf;a

例2

1. var_dump() 示例

代码如下

复制代码



<?php

$a = array (1, 2, array ("a", "b", "c"));

var_dump ($a);

/* 输出：
array(3) {
[0]=>
int(1)
[1]=>
int(2)
[2]=>
array(3) {
    [0]=>
    string(1) "a"
    [1]=>
    string(1) "b"
    [2]=>
    string(1) "c"
}
}

$b = 3.1;
$c = TRUE;
var_dump($b,$c);

/* 输出：
float(3.1)
bool(true)

*/
?>

var_export和serialize做数组缓存

$str = serialize($arr);
这两种机制转换之后的字符串是不一样的，第一种是数组的原型模式，第二种是序列化后的形式。第一种存入文件中的只要加上<?php ?>标签，就形式了一个可用的数组原型，对调用来说，不用转换，直接返回这个数组就可以，但第二种，则需要再用一次unserialize函数反序列化一下。对于第种一说，就多了一步操作。下来我们用数据说话吧:

代码如下

复制代码

<?
set_time_limit(50);
$a = array(1,2,3);
$b = array('a'=>1, 'b'=>2, 'c'=>3);
$c = array('a'=>array(1,2,3), 'b'=>array(4,5,6));

$time1 = microtime(true);

$times = 1000000; #10w

for($i=1; $i<=$times; $i++){
$A = var_export($a, true);
}

$time2 = microtime(true);

for($i=1; $i<=$times; $i++){
$B = var_export($b, true);
}

$time3 = microtime(true);

for($i=1; $i<=$times; $i++){
$C = var_export($c, true);
}

$time4 = microtime(true);

for($i=1; $i<=$times; $i++){
$X = serialize($a);
}

$time5 = microtime(true);

for($i=1; $i<=$times; $i++){
$Y = serialize($b);
}

$time6 = microtime(true);

for($i=1; $i<=$times; $i++){
$Z = serialize($c);
}

$time7 = microtime(true);

for($i=1; $i<=$times; $i++){
$O = unserialize($X);
}

$time8 = microtime(true);

for($i=1; $i<=$times; $i++){
$P = unserialize($Y);
}

$time9 = microtime(true);

for($i=1; $i<=$times; $i++){
$Q = unserialize($Z);
}
$time10 = microtime(true);

$var_export_time['a'] = $time2 - $time1;
$var_export_time['b'] = $time3 - $time2;
$var_export_time['c'] = $time4 - $time3;

$serialize_time['a'] = $time5 - $time4;
$serialize_time['b'] = $time6 - $time5;
$serialize_time['c'] = $time7 - $time6;

$unserialize_time['a'] = $time8 - $time7;
$unserialize_time['b'] = $time9 - $time8;
$unserialize_time['c'] = $time10 - $time9;
print_r($var_export_time);
print_r($serialize_time);
print_r($unserialize_time);
?>
output:
Array( [a] => 3.3401498794556 [b] => 5.1394801139832 [c] => 8.8483898639679)Array( [a] => 1.6063709259033 [b] => 1.7033960819244 [c] => 3.4534389972687)Array( [a] => 1.6037359237671 [b] => 1.817803144455 [c] => 3.7992968559265)

由上面数据说明：
var_export函数性能比serialize函数性能差一倍，而 unserialize时间也需要和serialize差不多的时间，serialize加上unserialize时间，和用var_export时间差不多。