Why is this an issue?

Using operator pairs (=+, =-, or =!) that look like reversed single operators (+=, -= or !=) is confusing. They compile and run but do not produce the same result as their mirrored counterpart.

$target = -5;
$num = 3;

$target =- $num;  // Noncompliant: target = -3. Is that the expected behavior?
$target =+ $num; // Noncompliant: target = 3

This rule raises an issue when =+, =-, or =! are used without any space between the operators and when there is at least one whitespace after.

Replace the operators with a single one if that is the intention

$target = -5;
$num = 3;

$target -= $num;  // target = -8

Or fix the spacing to avoid confusion

$target = -5;
$num = 3;

$target = -$num;  // target = -3