'', 'amps' => '', 'watts' => '', 'ohms' => '' ); // Clear zeroes. The above makes empty fields =0. if ($in['volts'] == '0'){ $in['volts'] = ''; }else{ $out['volts'] = $in['volts']; }; if ($in['amps'] == '0'){ $in['amps'] = ''; }else{ $out['amps'] = $in['amps']; }; if ($in['watts'] == '0'){ $in['watts'] = ''; }else{ $out['watts'] = $in['watts']; }; if ($in['ohms'] == '0'){ $in['ohms'] = ''; }else{ $out['ohms'] = $in['ohms']; }; // Process through all possible options until something is found, or nothing. // Run this a few times, just in case. Overkill, but may be useful later. $count = 0; while ($count <= 3){ $count++; // ===== VOLTS ================================== if ($out['volts'] == ''){ if ( ($in['amps'] != '') && ($in['ohms']) ){ $out['volts'] = $in['amps'] * $in['ohms']; }; if ($debug == 1){ if ($out['volts'] != ''){ print "Conversion [V1] V=I x R : " . $out['volts'] . " = " . $in['amps'] . "*" . $in['ohms'] .";
"; }; }; }; if ($out['volts'] == ''){ if ( ($in['watts'] != '') && ($in['amps']) ){ $out['volts'] = $in['watts'] / $in['amps']; }; if ($debug == 1){ if ($out['volts'] != ''){ print "Conversion [V2] V=P/R : ".$out['volts']." = ".$in['watts']." / ".$in['amps'].";
"; }; } }; if ($out['volts'] == ''){ if ( ($in['watts'] != '') && ($in['ohms']) ){ $out['volts'] = sqrt($in['watts'] * $in['ohms']); }; if ($debug == 1){ if ($out['volts'] != ''){ print "Conversion [V3] √ P x R : ".$out['volts']." = ".sqrt($in['watts']." * ".$in['ohms']).";
"; }; } }; // ===== AMPS ================================== if ($out['amps'] == ''){ if ( ($in['volts'] != '') && ($in['ohms']) ){ $out['amps'] = $in['volts'] / $in['ohms']; }; if ($debug == 1){ if ($out['amps'] != ''){ print "Conversion [A1] I = V⁄R : ". $out['amps']." = ".$in['volts']." / ".$in['ohms'].";
"; }; } }; if ($out['amps'] == ''){ if ( ($in['watts'] != '') && ($in['volts']) ){ $out['amps'] = $in['watts'] / $in['volts']; }; if ($debug == 1){ if ($out['amps'] != ''){ print "Conversion [A2] I = P⁄V : ". $out['amps']." = ".$in['watts']." / ".$in['volts'].";
"; }; } }; if ($out['amps'] == ''){ if ( ($in['watts'] != '') && ($in['ohms']) ){ $out['amps'] = sqrt($in['watts'] / $in['ohms']); }; if ($debug == 1){ if ($out['amps'] != ''){ print "Conversion [A3] I = √ P⁄R : ".$out['amps']." = sqrt(".$in['watts']." / ".$in['ohms'].");
"; }; } }; // ===== ['watts'] ================================== if ($out['watts'] == ''){ if ( ($in['volts'] != '') && ($in['amps'] != '') ){ $out['watts'] = $in['volts'] * $in['amps']; }; if ($debug == 1){ if ($out['watts'] != ''){ print "Conversion [W1] I = √ P⁄R : ".$out['amps']." = sqrt(".$in['watts']." / ".$in['ohms'].");
"; }; } }; if ($out['watts'] == ''){ if ( ($in['volts'] != '') && ($in['ohms'] != '') ){ $out['watts'] = pow($in['volts'],2) / $in['ohms']; }; if ($debug == 1){ if ($out['watts'] != ''){ print "Conversion [W2] P = V2⁄R : ".$out['watts']." = pow(".$in['volts'].",2) / ".$in['ohms'].";
"; } }; }; if ($out['watts'] == ''){ if ( ($in['amps'] != '') && ($in['ohms'] != '') ){ $out['watts'] = pow($in['amps'],2) * $in['ohms']; }; if ($debug == 1){ if ($out['watts'] != ''){ print "Conversion [W3] P = I2 x R : ".$out['watts']." = pow(".$in['amps'].",2) * ".$in['ohms'].";
"; }; } }; // ===== OHMS ================================== if ($out['ohms'] == ''){ if ( ($in['volts'] != '') && ($in['amps']) ){ $out['ohms'] = $in['volts'] / $in['amps']; }; if ($debug == 1){ if ($out['ohms'] != ''){ print "Conversion [R1] R = V⁄I :".$out['ohms']." = ".$in['volts']." / ".$in['amps'].";
"; }; } }; if ($out['ohms'] == ''){ if ( ($in['volts'] != '') && ($in['watts']) ){ $out['ohms'] = pow($in['volts'],2) / $in['watts']; }; if ($debug == 1){ if ($out['ohms'] != ''){ print "Conversion [R2] R = V2⁄P : ".$out['ohms']." = pow(".$in['volts'].",2) / ".$in['watts'].";
"; }; } }; if ($out['ohms'] == ''){ if ( ($in['watts'] != '') && ($in['amps']) ){ $out['ohms'] = $in['watts'] / pow($in['amps'],2); }; if ($debug == 1){ if ($out['ohms'] != ''){ print "Conversion [R3] R = P⁄I2 : ".$out['ohms']." = ".$in['watts']." / pow(".$in['amps'].",2);
"; }; } }; }; // Fix for PHP 8.0 if (is_numeric($out['volts'])){ $out['volts'] = round($out['volts'], 3); }; if (is_numeric($out['amps'])) { $out['amps'] = round($out['amps'], 3); }; if (is_numeric($out['watts'])){ $out['watts'] = round($out['watts'], 3); }; if (is_numeric($out['ohms'])) { $out['ohms'] = round($out['ohms'], 3); }; // Find our in/out highlighting colors. if ($in['volts'] == $out['volts']){ $out_hl['volts'] = "outfield"; }else{ $out_hl['volts'] = "outfieldcalc"; }; if ($in['amps'] == $out['amps']){ $out_hl['amps'] = "outfield"; }else{ $out_hl['amps'] = "outfieldcalc"; }; if ($in['watts'] == $out['watts']){ $out_hl['watts'] = "outfield"; }else{ $out_hl['watts'] = "outfieldcalc"; }; if ($in['ohms'] == $out['ohms']){ $out_hl['ohms'] = "outfield"; }else{ $out_hl['ohms'] = "outfieldcalc"; }; print " "; ?>

"; print "

Details
Voltage [V]	The measure of the power differental between two points. It is the electromotive force. This is measured in volts [V].
Current Iintensity [I]	The flow of electric charge across a surface. This is measured in amps [A].
Power [P]	The rate of energy transfer. This is measured in ['watts'] [W].
Resistance [R]	The rate of energy transfer. This is measured in ohms [Ω].

Formulas
To find	Known	Formula
Volts	Amps [I] Resistance Ω	V = I x R
Volts	Watts [P] Amps [I]	V = ^P⁄_I
Volts	Watts [P] Resistance Ω	√ P x R
Amps [I]	Volts Resistance Ω	I = ^V⁄_R
Amps [I]	Watts [P] Volts	I = ^P⁄_V
Amps [I]	Watts [P] Resistance Ω	I = √ ^P⁄_R
Watts [P]	Volts Amps [I]	P = V x I
Watts [P]	Volts Resistance Ω	P = ^V²⁄_R
Watts [P]	Amps [I] Resistance Ω	P = I² x R
Resistance Ω	Volts Amps [I]	R = ^V⁄_I
Resistance Ω	Volts Watts [P]	R = ^V²⁄_P
Resistance Ω	Watts [P] Amps [I]	R = ^P⁄_I²

^A⁄_B */ ?>

Further reference:

Revision history:

2020-12-02 : v1.4 - Updated code for PHP 8.0
- PHP Warning: Undefined variable $count
  defined.
- PHP Warning: Undefined array key "volts"
  predefined $out array.
- PHP Fatal error: Uncaught TypeError: round(): Argument #1 ($num) must be of type int|float, string given
  test is_numeric()
2020-12-01 : v1.3 - Fixed input bug. Watts field name mangled. Fixed.
2020-10-25 : v1.2 - Updated PHP, to correct for warnings for lazy coding
- "Use of undefined constant amps - assumed 'amps'"
- "syntax error, unexpected '' (T_ENCAPSED_AND_WHITESPACE),"
2017-07-29 : v1.1 - Fixed pow bug. x^2 != x*x . Use pow(x, 2).
Added if ($debug == 1), and index output of which formula(s) were used, with actual values.
2017-07-28 : v1.0 - Original work.