如何解释php中的“condition && $ var = value”语法？

I am inspecting a wordpress theme code and I found this row of code:

840 <= $width && $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px';

What does this row mean in terms of php?

To be more clear, this is the whole function:

/**
 * Add custom image sizes attribute to enhance responsive image functionality
 * for content images
 *
 * @since Twenty Sixteen 1.0
 *
 * @param string $sizes A source size value for use in a 'sizes' attribute.
 * @param array  $size  Image size. Accepts an array of width and height
 *                      values in pixels (in that order).
 * @return string A source size value for use in a content image 'sizes' attribute.
 */
function twentysixteen_content_image_sizes_attr( $sizes, $size ) {
    $width = $size[0];

    840 <= $width && $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px';

    if ( 'page' === get_post_type() ) {
        840 > $width && $sizes = '(max-width: ' . $width . 'px) 85vw, ' . $width . 'px';
    } else {
        840 > $width && 600 <= $width && $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 984px) 61vw, (max-width: 1362px) 45vw, 600px';
        600 > $width && $sizes = '(max-width: ' . $width . 'px) 85vw, ' . $width . 'px';
    }

    return $sizes;
}

This checks a condition and sets a variable in case it is true.

It is like saying:

conditionA && $variable="value";

In your case, it is like saying:

if (840 <= $width) {
    $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px';
}

See a little test:

<?php

$a=3;
$v=0;

echo "v = $v
";

$a == 3 && $v = "hello";

echo "v = $v";

If you execute it you get:

v = 0
v = hello

just like this:

840 <= $width && $sizes = '(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px';

is

conditionA && conditionB 

equal to

if( conditionA && conditionB ){} 

or

if( conditionA ) { conditionB } 

the same with

conditionA || conditionB

like

if( conditionA || conditionB ){}  

for example

$a = 1;
$b = 0;

// if( $a == 2 && $b = 3 ){}
$a == 2 && $b = 3; // && do not exec $b = 3,becuase $a != 2
echo $a,' ',$b;  // 1 0

// if( $a == 1 && $b = 4 ){}
$a == 1 && $b = 4; // exec $b = 4,becuase $a == 1
echo $a,' ',$b; //  1 4
// if( $a == 1 && $b = 4 ){}

// and how about  || 
$a = 1;
$b = 0;

// if( $a == 1 || $b = 3 ){}
$a == 1 || $b = 3; // && do not exec $b = 3,becuase $a == 1
echo $a,' ',$b; //  1 0

// if( $a == 2 || $b = 4 ){}
$a == 2 || $b = 4; // exec $b = 4,becuase $a == 1 not 2
echo $a,' ',$b;  // 1 4