php using eval to obtain an objects attribute value

Question

I am trying to set a function that will dynamically update an object attribute in the db w/o updating the whole object.

$sql = "UPDATE " . self::$table_name . " SET ";
$sql .= "$attribute = '" . eval("\$this->$attribute;") . "'";
$sql .= " WHERE ...";

I cant seem to get this eval("\$this->$attribute;") to produce the object attribute value. There is a value in the attribute and it is a public attribute.

Thanks

$attribute is a function var that will contain a string like 'address_id' so I want UPDATE table_name SET address_id = '11' WHERE user_id='1' This is a simple example of it

Why do you need to eval() it? Surely $this->attribute will suffice? — BenM
– BenM, Commented Feb 14, 2013 at 14:13
<cringe /> Why do you need to use eval in the first place? Why not simply use $this->attribute? — Mark Baker
– Mark Baker, Commented Feb 14, 2013 at 14:14
almost any time you use eval(), you're making a mistake. eval() is almost always unnecessary. It certainly doesn't seem necessary here. — SDC
– SDC, Commented Feb 14, 2013 at 14:27
$attribute is a function variable, tried '$this->attribute' didnt work. however I tried '$sql .= eval("return \$this->" . $attribute . ";");' and it worked. — user1747876
– user1747876, Commented Feb 14, 2013 at 14:29

MarcDefiant · Accepted Answer · 2013-02-14 14:23:20Z

2

You don't need to do this using eval. PHP supports variable variables: http://php.net/manual/en/language.variables.variable.php

So this will do:

$this->$attributes

Note the second $ sign. This basicly means the value of $attributes is used as attribute name. If you want it to write a bit clearer you can do so using brackets:

$this->{$attributes}

This bracket is required if you wanted to do this using an array, or if you wanted to use multiple variables to build a variable name like this:

$this->{$var1}_{$array[0]}_{$var2}

This goes probably behind the scope of this question, but its good to know what variable variables are. But i don't recommend using them, because it makes the code unreadable and hard to understand.

edited Feb 14, 2013 at 14:23

answered Feb 14, 2013 at 14:15

MarcDefiant

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BenM · Accepted Answer · 2013-02-14 14:24:10Z

1

There's no need to use eval():

$sql = "UPDATE " . self::$table_name . " SET ";
$sql .= "$attribute = '" . $this->attribute . "'";
$sql .= " WHERE ...";

If attribute is a variable, use:

$this->$attribute

Are you sure $this->attribute is sanitized correctly?

edited Feb 14, 2013 at 14:24

answered Feb 14, 2013 at 14:16

BenM

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7 Comments

Basic Bridge Over a year ago

I think it should be like this '".$this->attribute."'

BenM Over a year ago

Not if it's a variable variable.

MarcDefiant Over a year ago

you forgot the first dot . for the concatination

BenM Over a year ago

Then the above should be fine.

MarcDefiant Over a year ago

@BasicBridge BenM didn't get the concatination issue, i did, so i wrote it again so he understands what you meant.

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